bigIntConstAssumeCapacity

fn bigIntConstAssumeCapacity(
    self: *Builder,
    ty: Type,
    value: std.math.big.int.Const,
) Allocator.Error!Constant {
    const type_item = self.type_items.items[@backingInt(ty)];
    assert(type_item.tag == .integer);
    const bits = type_item.data;

    const ExpectedContents = [64 / @sizeOf(std.math.big.Limb)]std.math.big.Limb;
    var bfa_buf: ExpectedContents = undefined;
    var bfa: std.heap.BufferFirstAllocator = .init(@ptrCast(&bfa_buf), self.gpa);
    const allocator = bfa.allocator();

    var limbs: []std.math.big.Limb = &.{};
    defer allocator.free(limbs);
    const canonical_value = if (value.fitsInTwosComp(.signed, bits)) value else canon: {
        assert(value.fitsInTwosComp(.unsigned, bits));
        limbs = try allocator.alloc(std.math.big.Limb, std.math.big.int.calcTwosCompLimbCount(bits));
        var temp_value = std.math.big.int.Mutable.init(limbs, 0);
        temp_value.truncate(value, .signed, bits);
        break :canon temp_value.toConst();
    };
    assert(canonical_value.fitsInTwosComp(.signed, bits));

    const ExtraPtr = *align(@alignOf(std.math.big.Limb)) Constant.Integer;
    const Key = struct { tag: Constant.Tag, type: Type, limbs: []const std.math.big.Limb };
    const tag: Constant.Tag = switch (canonical_value.positive) {
        true => .positive_integer,
        false => .negative_integer,
    };
    const Adapter = struct {
        builder: *const Builder,
        pub fn hash(_: @This(), key: Key) u32 {
            var hasher = std.hash.Wyhash.init(std.hash.int(@backingInt(key.tag)));
            hasher.update(std.mem.asBytes(&key.type));
            hasher.update(std.mem.sliceAsBytes(key.limbs));
            return @truncate(hasher.final());
        }
        pub fn eql(ctx: @This(), lhs_key: Key, _: void, rhs_index: usize) bool {
            if (lhs_key.tag != ctx.builder.constant_items.items(.tag)[rhs_index]) return false;
            const rhs_data = ctx.builder.constant_items.items(.data)[rhs_index];
            const rhs_extra: ExtraPtr =
                @ptrCast(ctx.builder.constant_limbs.items[rhs_data..][0..Constant.Integer.limbs]);
            const rhs_limbs = ctx.builder.constant_limbs
                .items[rhs_data + Constant.Integer.limbs ..][0..rhs_extra.limbs_len];
            return lhs_key.type == rhs_extra.type and
                std.mem.eql(std.math.big.Limb, lhs_key.limbs, rhs_limbs);
        }
    };

    const gop = self.constant_map.getOrPutAssumeCapacityAdapted(
        Key{ .tag = tag, .type = ty, .limbs = canonical_value.limbs },
        Adapter{ .builder = self },
    );
    if (!gop.found_existing) {
        gop.key_ptr.* = {};
        gop.value_ptr.* = {};
        self.constant_items.appendAssumeCapacity(.{
            .tag = tag,
            .data = @intCast(self.constant_limbs.items.len),
        });
        const extra: ExtraPtr =
            @ptrCast(self.constant_limbs.addManyAsArrayAssumeCapacity(Constant.Integer.limbs));
        extra.* = .{ .type = ty, .limbs_len = @intCast(canonical_value.limbs.len) };
        self.constant_limbs.appendSliceAssumeCapacity(canonical_value.limbs);
    }
    return @fromBackingInt(@intCast(gop.index));
}