std.enums

pub fn fromInt(comptime E: type, integer: anytype) ?E {
    const enum_info = @typeInfo(E).@"enum";
    if (!enum_info.is_exhaustive) {
        if (std.math.cast(enum_info.tag_type, integer)) |tag| {
            return @enumFromInt(tag);
        }
        return null;
    }
    // We don't directly iterate over the fields of E, as that
    // would require an inline loop. Instead, we create an array of
    // values that is comptime-know, but can be iterated at runtime
    // without requiring an inline loop.
    // This generates better machine code.
    for (values(E)) |value| {
        if (@intFromEnum(value) == integer) return @enumFromInt(integer);
    }
    return null;
}

pub fn EnumFieldStruct(comptime E: type, comptime Data: type, comptime field_default: ?Data) type {
    @setEvalBranchQuota(@typeInfo(E).@"enum".fields.len + eval_branch_quota_cushion);
    var struct_fields: [@typeInfo(E).@"enum".fields.len]std.builtin.Type.StructField = undefined;
    for (&struct_fields, @typeInfo(E).@"enum".fields) |*struct_field, enum_field| {
        struct_field.* = .{
            .name = enum_field.name,
            .type = Data,
            .default_value_ptr = if (field_default) |d| @as(?*const anyopaque, @ptrCast(&d)) else null,
            .is_comptime = false,
            .alignment = if (@sizeOf(Data) > 0) @alignOf(Data) else 0,
        };
    }
    return @Type(.{ .@"struct" = .{
        .layout = .auto,
        .fields = &struct_fields,
        .decls = &.{},
        .is_tuple = false,
    } });
}

pub inline fn valuesFromFields(comptime E: type, comptime fields: []const EnumField) []const E {
    comptime {
        var result: [fields.len]E = undefined;
        for (&result, fields) |*r, f| {
            r.* = @enumFromInt(f.value);
        }
        const final = result;
        return &final;
    }
}

pub fn values(comptime E: type) []const E {
    return comptime valuesFromFields(E, @typeInfo(E).@"enum".fields);
}

pub fn tagName(comptime E: type, e: E) ?[:0]const u8 {
    return inline for (@typeInfo(E).@"enum".fields) |f| {
        if (@intFromEnum(e) == f.value) break f.name;
    } else null;
}

pub fn directEnumArrayLen(comptime E: type, comptime max_unused_slots: comptime_int) comptime_int {
    var max_value: comptime_int = -1;
    const max_usize: comptime_int = ~@as(usize, 0);
    const fields = @typeInfo(E).@"enum".fields;
    for (fields) |f| {
        if (f.value < 0) {
            @compileError("Cannot create a direct enum array for " ++ @typeName(E) ++ ", field ." ++ f.name ++ " has a negative value.");
        }
        if (f.value > max_value) {
            if (f.value > max_usize) {
                @compileError("Cannot create a direct enum array for " ++ @typeName(E) ++ ", field ." ++ f.name ++ " is larger than the max value of usize.");
            }
            max_value = f.value;
        }
    }

    const unused_slots = max_value + 1 - fields.len;
    if (unused_slots > max_unused_slots) {
        const unused_str = std.fmt.comptimePrint("{d}", .{unused_slots});
        const allowed_str = std.fmt.comptimePrint("{d}", .{max_unused_slots});
        @compileError("Cannot create a direct enum array for " ++ @typeName(E) ++ ". It would have " ++ unused_str ++ " unused slots, but only " ++ allowed_str ++ " are allowed.");
    }

    return max_value + 1;
}

pub fn directEnumArray(
    comptime E: type,
    comptime Data: type,
    comptime max_unused_slots: comptime_int,
    init_values: EnumFieldStruct(E, Data, null),
) [directEnumArrayLen(E, max_unused_slots)]Data {
    return directEnumArrayDefault(E, Data, null, max_unused_slots, init_values);
}

pub fn directEnumArrayDefault(
    comptime E: type,
    comptime Data: type,
    comptime default: ?Data,
    comptime max_unused_slots: comptime_int,
    init_values: EnumFieldStruct(E, Data, default),
) [directEnumArrayLen(E, max_unused_slots)]Data {
    const len = comptime directEnumArrayLen(E, max_unused_slots);
    var result: [len]Data = if (default) |d| [_]Data{d} ** len else undefined;
    inline for (@typeInfo(@TypeOf(init_values)).@"struct".fields) |f| {
        const enum_value = @field(E, f.name);
        const index = @as(usize, @intCast(@intFromEnum(enum_value)));
        result[index] = @field(init_values, f.name);
    }
    return result;
}

pub fn nameCast(comptime E: type, comptime value: anytype) E {
    return comptime blk: {
        const V = @TypeOf(value);
        if (V == E) break :blk value;
        const name: ?[]const u8 = switch (@typeInfo(V)) {
            .enum_literal, .@"enum" => @tagName(value),
            .pointer => value,
            else => null,
        };
        if (name) |n| {
            if (@hasField(E, n)) {
                break :blk @field(E, n);
            }
            @compileError("Enum " ++ @typeName(E) ++ " has no field named " ++ n);
        }
        @compileError("Cannot cast from " ++ @typeName(@TypeOf(value)) ++ " to " ++ @typeName(E));
    };
}

pub fn EnumSet(comptime E: type) type {
    return struct {
        const Self = @This();

        /// The indexing rules for converting between keys and indices.
        pub const Indexer = EnumIndexer(E);
        /// The element type for this set.
        pub const Key = Indexer.Key;

        const BitSet = std.StaticBitSet(Indexer.count);

        /// The maximum number of items in this set.
        pub const len = Indexer.count;

        bits: BitSet = BitSet.initEmpty(),

        /// Initializes the set using a struct of bools
        pub fn init(init_values: EnumFieldStruct(E, bool, false)) Self {
            @setEvalBranchQuota(2 * @typeInfo(E).@"enum".fields.len);
            var result: Self = .{};
            if (@typeInfo(E).@"enum".is_exhaustive) {
                inline for (0..Self.len) |i| {
                    const key = comptime Indexer.keyForIndex(i);
                    const tag = @tagName(key);
                    if (@field(init_values, tag)) {
                        result.bits.set(i);
                    }
                }
            } else {
                inline for (std.meta.fields(E)) |field| {
                    const key = @field(E, field.name);
                    if (@field(init_values, field.name)) {
                        const i = comptime Indexer.indexOf(key);
                        result.bits.set(i);
                    }
                }
            }
            return result;
        }

        /// Returns a set containing no keys.
        pub fn initEmpty() Self {
            return .{ .bits = BitSet.initEmpty() };
        }

        /// Returns a set containing all possible keys.
        pub fn initFull() Self {
            return .{ .bits = BitSet.initFull() };
        }

        /// Returns a set containing multiple keys.
        pub fn initMany(keys: []const Key) Self {
            var set = initEmpty();
            for (keys) |key| set.insert(key);
            return set;
        }

        /// Returns a set containing a single key.
        pub fn initOne(key: Key) Self {
            return initMany(&[_]Key{key});
        }

        /// Returns the number of keys in the set.
        pub fn count(self: Self) usize {
            return self.bits.count();
        }

        /// Checks if a key is in the set.
        pub fn contains(self: Self, key: Key) bool {
            return self.bits.isSet(Indexer.indexOf(key));
        }

        /// Puts a key in the set.
        pub fn insert(self: *Self, key: Key) void {
            self.bits.set(Indexer.indexOf(key));
        }

        /// Removes a key from the set.
        pub fn remove(self: *Self, key: Key) void {
            self.bits.unset(Indexer.indexOf(key));
        }

        /// Changes the presence of a key in the set to match the passed bool.
        pub fn setPresent(self: *Self, key: Key, present: bool) void {
            self.bits.setValue(Indexer.indexOf(key), present);
        }

        /// Toggles the presence of a key in the set.  If the key is in
        /// the set, removes it.  Otherwise adds it.
        pub fn toggle(self: *Self, key: Key) void {
            self.bits.toggle(Indexer.indexOf(key));
        }

        /// Toggles the presence of all keys in the passed set.
        pub fn toggleSet(self: *Self, other: Self) void {
            self.bits.toggleSet(other.bits);
        }

        /// Toggles all possible keys in the set.
        pub fn toggleAll(self: *Self) void {
            self.bits.toggleAll();
        }

        /// Adds all keys in the passed set to this set.
        pub fn setUnion(self: *Self, other: Self) void {
            self.bits.setUnion(other.bits);
        }

        /// Removes all keys which are not in the passed set.
        pub fn setIntersection(self: *Self, other: Self) void {
            self.bits.setIntersection(other.bits);
        }

        /// Returns true iff both sets have the same keys.
        pub fn eql(self: Self, other: Self) bool {
            return self.bits.eql(other.bits);
        }

        /// Returns true iff all the keys in this set are
        /// in the other set. The other set may have keys
        /// not found in this set.
        pub fn subsetOf(self: Self, other: Self) bool {
            return self.bits.subsetOf(other.bits);
        }

        /// Returns true iff this set contains all the keys
        /// in the other set. This set may have keys not
        /// found in the other set.
        pub fn supersetOf(self: Self, other: Self) bool {
            return self.bits.supersetOf(other.bits);
        }

        /// Returns a set with all the keys not in this set.
        pub fn complement(self: Self) Self {
            return .{ .bits = self.bits.complement() };
        }

        /// Returns a set with keys that are in either this
        /// set or the other set.
        pub fn unionWith(self: Self, other: Self) Self {
            return .{ .bits = self.bits.unionWith(other.bits) };
        }

        /// Returns a set with keys that are in both this
        /// set and the other set.
        pub fn intersectWith(self: Self, other: Self) Self {
            return .{ .bits = self.bits.intersectWith(other.bits) };
        }

        /// Returns a set with keys that are in either this
        /// set or the other set, but not both.
        pub fn xorWith(self: Self, other: Self) Self {
            return .{ .bits = self.bits.xorWith(other.bits) };
        }

        /// Returns a set with keys that are in this set
        /// except for keys in the other set.
        pub fn differenceWith(self: Self, other: Self) Self {
            return .{ .bits = self.bits.differenceWith(other.bits) };
        }

        /// Returns an iterator over this set, which iterates in
        /// index order.  Modifications to the set during iteration
        /// may or may not be observed by the iterator, but will
        /// not invalidate it.
        pub fn iterator(self: *const Self) Iterator {
            return .{ .inner = self.bits.iterator(.{}) };
        }

        pub const Iterator = struct {
            inner: BitSet.Iterator(.{}),

            pub fn next(self: *Iterator) ?Key {
                return if (self.inner.next()) |index|
                    Indexer.keyForIndex(index)
                else
                    null;
            }
        };
    };
}

pub fn EnumMap(comptime E: type, comptime V: type) type {
    return struct {
        const Self = @This();

        /// The index mapping for this map
        pub const Indexer = EnumIndexer(E);
        /// The key type used to index this map
        pub const Key = Indexer.Key;
        /// The value type stored in this map
        pub const Value = V;
        /// The number of possible keys in the map
        pub const len = Indexer.count;

        const BitSet = std.StaticBitSet(Indexer.count);

        /// Bits determining whether items are in the map
        bits: BitSet = BitSet.initEmpty(),
        /// Values of items in the map.  If the associated
        /// bit is zero, the value is undefined.
        values: [Indexer.count]Value = undefined,

        /// Initializes the map using a sparse struct of optionals
        pub fn init(init_values: EnumFieldStruct(E, ?Value, @as(?Value, null))) Self {
            @setEvalBranchQuota(2 * @typeInfo(E).@"enum".fields.len);
            var result: Self = .{};
            if (@typeInfo(E).@"enum".is_exhaustive) {
                inline for (0..Self.len) |i| {
                    const key = comptime Indexer.keyForIndex(i);
                    const tag = @tagName(key);
                    if (@field(init_values, tag)) |*v| {
                        result.bits.set(i);
                        result.values[i] = v.*;
                    }
                }
            } else {
                inline for (std.meta.fields(E)) |field| {
                    const key = @field(E, field.name);
                    if (@field(init_values, field.name)) |*v| {
                        const i = comptime Indexer.indexOf(key);
                        result.bits.set(i);
                        result.values[i] = v.*;
                    }
                }
            }
            return result;
        }

        /// Initializes a full mapping with all keys set to value.
        /// Consider using EnumArray instead if the map will remain full.
        pub fn initFull(value: Value) Self {
            var result: Self = .{
                .bits = Self.BitSet.initFull(),
                .values = undefined,
            };
            @memset(&result.values, value);
            return result;
        }

        /// Initializes a full mapping with supplied values.
        /// Consider using EnumArray instead if the map will remain full.
        pub fn initFullWith(init_values: EnumFieldStruct(E, Value, null)) Self {
            return initFullWithDefault(null, init_values);
        }

        /// Initializes a full mapping with a provided default.
        /// Consider using EnumArray instead if the map will remain full.
        pub fn initFullWithDefault(comptime default: ?Value, init_values: EnumFieldStruct(E, Value, default)) Self {
            @setEvalBranchQuota(2 * @typeInfo(E).@"enum".fields.len);
            var result: Self = .{
                .bits = Self.BitSet.initFull(),
                .values = undefined,
            };
            inline for (0..Self.len) |i| {
                const key = comptime Indexer.keyForIndex(i);
                const tag = @tagName(key);
                result.values[i] = @field(init_values, tag);
            }
            return result;
        }

        /// The number of items in the map.
        pub fn count(self: Self) usize {
            return self.bits.count();
        }

        /// Checks if the map contains an item.
        pub fn contains(self: Self, key: Key) bool {
            return self.bits.isSet(Indexer.indexOf(key));
        }

        /// Gets the value associated with a key.
        /// If the key is not in the map, returns null.
        pub fn get(self: Self, key: Key) ?Value {
            const index = Indexer.indexOf(key);
            return if (self.bits.isSet(index)) self.values[index] else null;
        }

        /// Gets the value associated with a key, which must
        /// exist in the map.
        pub fn getAssertContains(self: Self, key: Key) Value {
            const index = Indexer.indexOf(key);
            assert(self.bits.isSet(index));
            return self.values[index];
        }

        /// Gets the address of the value associated with a key.
        /// If the key is not in the map, returns null.
        pub fn getPtr(self: *Self, key: Key) ?*Value {
            const index = Indexer.indexOf(key);
            return if (self.bits.isSet(index)) &self.values[index] else null;
        }

        /// Gets the address of the const value associated with a key.
        /// If the key is not in the map, returns null.
        pub fn getPtrConst(self: *const Self, key: Key) ?*const Value {
            const index = Indexer.indexOf(key);
            return if (self.bits.isSet(index)) &self.values[index] else null;
        }

        /// Gets the address of the value associated with a key.
        /// The key must be present in the map.
        pub fn getPtrAssertContains(self: *Self, key: Key) *Value {
            const index = Indexer.indexOf(key);
            assert(self.bits.isSet(index));
            return &self.values[index];
        }

        /// Gets the address of the const value associated with a key.
        /// The key must be present in the map.
        pub fn getPtrConstAssertContains(self: *const Self, key: Key) *const Value {
            const index = Indexer.indexOf(key);
            assert(self.bits.isSet(index));
            return &self.values[index];
        }

        /// Adds the key to the map with the supplied value.
        /// If the key is already in the map, overwrites the value.
        pub fn put(self: *Self, key: Key, value: Value) void {
            const index = Indexer.indexOf(key);
            self.bits.set(index);
            self.values[index] = value;
        }

        /// Adds the key to the map with an undefined value.
        /// If the key is already in the map, the value becomes undefined.
        /// A pointer to the value is returned, which should be
        /// used to initialize the value.
        pub fn putUninitialized(self: *Self, key: Key) *Value {
            const index = Indexer.indexOf(key);
            self.bits.set(index);
            self.values[index] = undefined;
            return &self.values[index];
        }

        /// Sets the value associated with the key in the map,
        /// and returns the old value.  If the key was not in
        /// the map, returns null.
        pub fn fetchPut(self: *Self, key: Key, value: Value) ?Value {
            const index = Indexer.indexOf(key);
            const result: ?Value = if (self.bits.isSet(index)) self.values[index] else null;
            self.bits.set(index);
            self.values[index] = value;
            return result;
        }

        /// Removes a key from the map.  If the key was not in the map,
        /// does nothing.
        pub fn remove(self: *Self, key: Key) void {
            const index = Indexer.indexOf(key);
            self.bits.unset(index);
            self.values[index] = undefined;
        }

        /// Removes a key from the map, and returns the old value.
        /// If the key was not in the map, returns null.
        pub fn fetchRemove(self: *Self, key: Key) ?Value {
            const index = Indexer.indexOf(key);
            const result: ?Value = if (self.bits.isSet(index)) self.values[index] else null;
            self.bits.unset(index);
            self.values[index] = undefined;
            return result;
        }

        /// Returns an iterator over the map, which visits items in index order.
        /// Modifications to the underlying map may or may not be observed by
        /// the iterator, but will not invalidate it.
        pub fn iterator(self: *Self) Iterator {
            return .{
                .inner = self.bits.iterator(.{}),
                .values = &self.values,
            };
        }

        /// An entry in the map.
        pub const Entry = struct {
            /// The key associated with this entry.
            /// Modifying this key will not change the map.
            key: Key,

            /// A pointer to the value in the map associated
            /// with this key.  Modifications through this
            /// pointer will modify the underlying data.
            value: *Value,
        };

        pub const Iterator = struct {
            inner: BitSet.Iterator(.{}),
            values: *[Indexer.count]Value,

            pub fn next(self: *Iterator) ?Entry {
                return if (self.inner.next()) |index|
                    Entry{
                        .key = Indexer.keyForIndex(index),
                        .value = &self.values[index],
                    }
                else
                    null;
            }
        };
    };
}

pub fn EnumMultiset(comptime E: type) type {
    return BoundedEnumMultiset(E, usize);
}

pub fn BoundedEnumMultiset(comptime E: type, comptime CountSize: type) type {
    return struct {
        const Self = @This();

        counts: EnumArray(E, CountSize),

        /// Initializes the multiset using a struct of counts.
        pub fn init(init_counts: EnumFieldStruct(E, CountSize, 0)) Self {
            @setEvalBranchQuota(2 * @typeInfo(E).@"enum".fields.len);
            var self = initWithCount(0);
            inline for (@typeInfo(E).@"enum".fields) |field| {
                const c = @field(init_counts, field.name);
                const key = @as(E, @enumFromInt(field.value));
                self.counts.set(key, c);
            }
            return self;
        }

        /// Initializes the multiset with a count of zero.
        pub fn initEmpty() Self {
            return initWithCount(0);
        }

        /// Initializes the multiset with all keys at the
        /// same count.
        pub fn initWithCount(comptime c: CountSize) Self {
            return .{
                .counts = EnumArray(E, CountSize).initDefault(c, .{}),
            };
        }

        /// Returns the total number of key counts in the multiset.
        pub fn count(self: Self) usize {
            var sum: usize = 0;
            for (self.counts.values) |c| {
                sum += c;
            }
            return sum;
        }

        /// Checks if at least one key in multiset.
        pub fn contains(self: Self, key: E) bool {
            return self.counts.get(key) > 0;
        }

        /// Removes all instance of a key from multiset. Same as
        /// setCount(key, 0).
        pub fn removeAll(self: *Self, key: E) void {
            return self.counts.set(key, 0);
        }

        /// Increases the key count by given amount. Caller asserts
        /// operation will not overflow.
        pub fn addAssertSafe(self: *Self, key: E, c: CountSize) void {
            self.counts.getPtr(key).* += c;
        }

        /// Increases the key count by given amount.
        pub fn add(self: *Self, key: E, c: CountSize) error{Overflow}!void {
            self.counts.set(key, try std.math.add(CountSize, self.counts.get(key), c));
        }

        /// Decreases the key count by given amount. If amount is
        /// greater than the number of keys in multset, then key count
        /// will be set to zero.
        pub fn remove(self: *Self, key: E, c: CountSize) void {
            self.counts.getPtr(key).* -= @min(self.getCount(key), c);
        }

        /// Returns the count for a key.
        pub fn getCount(self: Self, key: E) CountSize {
            return self.counts.get(key);
        }

        /// Set the count for a key.
        pub fn setCount(self: *Self, key: E, c: CountSize) void {
            self.counts.set(key, c);
        }

        /// Increases the all key counts by given multiset. Caller
        /// asserts operation will not overflow any key.
        pub fn addSetAssertSafe(self: *Self, other: Self) void {
            inline for (@typeInfo(E).@"enum".fields) |field| {
                const key = @as(E, @enumFromInt(field.value));
                self.addAssertSafe(key, other.getCount(key));
            }
        }

        /// Increases the all key counts by given multiset.
        pub fn addSet(self: *Self, other: Self) error{Overflow}!void {
            inline for (@typeInfo(E).@"enum".fields) |field| {
                const key = @as(E, @enumFromInt(field.value));
                try self.add(key, other.getCount(key));
            }
        }

        /// Decreases the all key counts by given multiset. If
        /// the given multiset has more key counts than this,
        /// then that key will have a key count of zero.
        pub fn removeSet(self: *Self, other: Self) void {
            inline for (@typeInfo(E).@"enum".fields) |field| {
                const key = @as(E, @enumFromInt(field.value));
                self.remove(key, other.getCount(key));
            }
        }

        /// Returns true iff all key counts are the same as
        /// given multiset.
        pub fn eql(self: Self, other: Self) bool {
            inline for (@typeInfo(E).@"enum".fields) |field| {
                const key = @as(E, @enumFromInt(field.value));
                if (self.getCount(key) != other.getCount(key)) {
                    return false;
                }
            }
            return true;
        }

        /// Returns true iff all key counts less than or
        /// equal to the given multiset.
        pub fn subsetOf(self: Self, other: Self) bool {
            inline for (@typeInfo(E).@"enum".fields) |field| {
                const key = @as(E, @enumFromInt(field.value));
                if (self.getCount(key) > other.getCount(key)) {
                    return false;
                }
            }
            return true;
        }

        /// Returns true iff all key counts greater than or
        /// equal to the given multiset.
        pub fn supersetOf(self: Self, other: Self) bool {
            inline for (@typeInfo(E).@"enum".fields) |field| {
                const key = @as(E, @enumFromInt(field.value));
                if (self.getCount(key) < other.getCount(key)) {
                    return false;
                }
            }
            return true;
        }

        /// Returns a multiset with the total key count of this
        /// multiset and the other multiset. Caller asserts
        /// operation will not overflow any key.
        pub fn plusAssertSafe(self: Self, other: Self) Self {
            var result = self;
            result.addSetAssertSafe(other);
            return result;
        }

        /// Returns a multiset with the total key count of this
        /// multiset and the other multiset.
        pub fn plus(self: Self, other: Self) error{Overflow}!Self {
            var result = self;
            try result.addSet(other);
            return result;
        }

        /// Returns a multiset with the key count of this
        /// multiset minus the corresponding key count in the
        /// other multiset. If the other multiset contains
        /// more key count than this set, that key will have
        /// a count of zero.
        pub fn minus(self: Self, other: Self) Self {
            var result = self;
            result.removeSet(other);
            return result;
        }

        pub const Entry = EnumArray(E, CountSize).Entry;
        pub const Iterator = EnumArray(E, CountSize).Iterator;

        /// Returns an iterator over this multiset. Keys with zero
        /// counts are included. Modifications to the set during
        /// iteration may or may not be observed by the iterator,
        /// but will not invalidate it.
        pub fn iterator(self: *Self) Iterator {
            return self.counts.iterator();
        }
    };
}

pub fn EnumArray(comptime E: type, comptime V: type) type {
    return struct {
        const Self = @This();

        /// The index mapping for this map
        pub const Indexer = EnumIndexer(E);
        /// The key type used to index this map
        pub const Key = Indexer.Key;
        /// The value type stored in this map
        pub const Value = V;
        /// The number of possible keys in the map
        pub const len = Indexer.count;

        values: [Indexer.count]Value,

        pub fn init(init_values: EnumFieldStruct(E, Value, null)) Self {
            return initDefault(null, init_values);
        }

        /// Initializes values in the enum array, with the specified default.
        pub fn initDefault(comptime default: ?Value, init_values: EnumFieldStruct(E, Value, default)) Self {
            @setEvalBranchQuota(2 * @typeInfo(E).@"enum".fields.len);
            var result: Self = .{ .values = undefined };
            inline for (0..Self.len) |i| {
                const key = comptime Indexer.keyForIndex(i);
                const tag = @tagName(key);
                result.values[i] = @field(init_values, tag);
            }
            return result;
        }

        pub fn initUndefined() Self {
            return Self{ .values = undefined };
        }

        pub fn initFill(v: Value) Self {
            var self: Self = undefined;
            @memset(&self.values, v);
            return self;
        }

        /// Returns the value in the array associated with a key.
        pub fn get(self: Self, key: Key) Value {
            return self.values[Indexer.indexOf(key)];
        }

        /// Returns a pointer to the slot in the array associated with a key.
        pub fn getPtr(self: *Self, key: Key) *Value {
            return &self.values[Indexer.indexOf(key)];
        }

        /// Returns a const pointer to the slot in the array associated with a key.
        pub fn getPtrConst(self: *const Self, key: Key) *const Value {
            return &self.values[Indexer.indexOf(key)];
        }

        /// Sets the value in the slot associated with a key.
        pub fn set(self: *Self, key: Key, value: Value) void {
            self.values[Indexer.indexOf(key)] = value;
        }

        /// Iterates over the items in the array, in index order.
        pub fn iterator(self: *Self) Iterator {
            return .{
                .values = &self.values,
            };
        }

        /// An entry in the array.
        pub const Entry = struct {
            /// The key associated with this entry.
            /// Modifying this key will not change the array.
            key: Key,

            /// A pointer to the value in the array associated
            /// with this key.  Modifications through this
            /// pointer will modify the underlying data.
            value: *Value,
        };

        pub const Iterator = struct {
            index: usize = 0,
            values: *[Indexer.count]Value,

            pub fn next(self: *Iterator) ?Entry {
                const index = self.index;
                if (index < Indexer.count) {
                    self.index += 1;
                    return Entry{
                        .key = Indexer.keyForIndex(index),
                        .value = &self.values[index],
                    };
                }
                return null;
            }
        };
    };
}

pub fn EnumIndexer(comptime E: type) type {
    // n log n for `std.mem.sortUnstable` call below.
    const fields_len = @typeInfo(E).@"enum".fields.len;
    @setEvalBranchQuota(3 * fields_len * std.math.log2(@max(fields_len, 1)) + eval_branch_quota_cushion);

    if (!@typeInfo(E).@"enum".is_exhaustive) {
        const BackingInt = @typeInfo(E).@"enum".tag_type;
        if (@bitSizeOf(BackingInt) > @bitSizeOf(usize))
            @compileError("Cannot create an enum indexer for a given non-exhaustive enum, tag_type is larger than usize.");

        return struct {
            pub const Key: type = E;

            const backing_int_sign = @typeInfo(BackingInt).int.signedness;
            const min_value = std.math.minInt(BackingInt);
            const max_value = std.math.maxInt(BackingInt);

            const RangeType = std.meta.Int(.unsigned, @bitSizeOf(BackingInt));
            pub const count: comptime_int = std.math.maxInt(RangeType) + 1;

            pub fn indexOf(e: E) usize {
                if (backing_int_sign == .unsigned)
                    return @intFromEnum(e);

                return if (@intFromEnum(e) < 0)
                    @intCast(@intFromEnum(e) - min_value)
                else
                    @as(RangeType, -min_value) + @as(RangeType, @intCast(@intFromEnum(e)));
            }
            pub fn keyForIndex(i: usize) E {
                if (backing_int_sign == .unsigned)
                    return @enumFromInt(i);

                return @enumFromInt(@as(std.meta.Int(.signed, @bitSizeOf(RangeType) + 1), @intCast(i)) + min_value);
            }
        };
    }

    if (fields_len == 0) {
        return struct {
            pub const Key = E;
            pub const count: comptime_int = 0;
            pub fn indexOf(e: E) usize {
                _ = e;
                unreachable;
            }
            pub fn keyForIndex(i: usize) E {
                _ = i;
                unreachable;
            }
        };
    }

    var fields: [fields_len]EnumField = @typeInfo(E).@"enum".fields[0..].*;

    std.mem.sortUnstable(EnumField, &fields, {}, struct {
        fn lessThan(ctx: void, lhs: EnumField, rhs: EnumField) bool {
            ctx;
            return lhs.value < rhs.value;
        }
    }.lessThan);

    const min = fields[0].value;
    const max = fields[fields_len - 1].value;
    if (max - min == fields.len - 1) {
        return struct {
            pub const Key = E;
            pub const count: comptime_int = fields_len;
            pub fn indexOf(e: E) usize {
                return @as(usize, @intCast(@intFromEnum(e) - min));
            }
            pub fn keyForIndex(i: usize) E {
                // TODO fix addition semantics.  This calculation
                // gives up some safety to avoid artificially limiting
                // the range of signed enum values to max_isize.
                const enum_value = if (min < 0) @as(isize, @bitCast(i)) +% min else i + min;
                return @as(E, @enumFromInt(@as(@typeInfo(E).@"enum".tag_type, @intCast(enum_value))));
            }
        };
    }

    const keys = valuesFromFields(E, &fields);

    return struct {
        pub const Key = E;
        pub const count: comptime_int = fields_len;
        pub fn indexOf(e: E) usize {
            for (keys, 0..) |k, i| {
                if (k == e) return i;
            }
            unreachable;
        }
        pub fn keyForIndex(i: usize) E {
            return keys[i];
        }
    };
}