const io = @import("./io.zig");

const math = @import("./math.zig");

pub const DecimalFormat = struct {
	delimiter: []const io.Byte,
	positive_prefix: enum {none, plus, space},

	pub fn parse(self: DecimalFormat, utf8: []const io.Byte, comptime Decimal: type) ?Decimal {
		if (utf8.len == 0) {
			return null;
		}

		switch (@typeInfo(Decimal)) {
			.Int => |int| {
				var has_sign = switch (utf8[0]) {
					'-', '+', ' ' => true,
					else => false,
				};

				var result = @as(Decimal, 0);

				for (@intFromBool(has_sign) .. utf8.len) |index| {
					const radix = 10;
					const code = utf8[index];

					switch (code) {
						'0', '1', '2', '3', '4', '5', '6', '7', '8', '9' => {
							result = try math.checked_add(
								try math.checked_mul(result, radix),
								try math.checked_sub(code, '0'));
						},

						else => {
							if (self.delimiter.len == 0 or !io.equals(self.delimiter, utf8[index ..])) {
								return null;
							}
						},
					}
				}

				switch (int.signedness) {
					.signed => {
						return result * @as(Decimal, if (has_sign and utf8[0] == '-') -1 else 1);
					},

					.unsigned => {
						if (has_sign and utf8[0] == '-') {
							return null;
						}

						return result;
					},
				}
			},

			.Float => {
				var has_sign = switch (utf8[0]) {
					'-', '+', ' ' => true,
					else => false,
				};

				// "-"
				if (has_sign and utf8.len == 1) {
					return null;
				}

				const sign_offset = @intFromBool(has_sign);
				var has_decimal = utf8[sign_offset] == '.';

				// "-."
				if (has_decimal and (utf8.len == 2)) {
					return null;
				}

				var result = @as(Decimal, 0);
				var factor = @as(Decimal, if (has_sign and utf8[0] == '-') -1 else 1);

				for (utf8[sign_offset + @intFromBool(has_decimal) .. utf8.len]) |code| {
					switch (code) {
						'.' => {
							if (has_decimal) {
								return null;
							}

							has_decimal = true;
						},

						'0', '1', '2', '3', '4', '5', '6', '7', '8', '9' => {
							if (has_decimal) {
								factor /= 10.0;
							}

							result = ((result * 10.0) + @as(Decimal, @floatFromInt(code - '0')));
						},

						else => return null,
					}
				}

				return result * factor;
			},

			else => @compileError("`" ++ @typeName(Decimal) ++ "` cannot be formatted as a decimal string"),
		}
	}
};