    const std = @import("../std.zig");
    const enum3 = @import("errol/enum3.zig").enum3;
    const enum3_data = @import("errol/enum3.zig").enum3_data;
    const lookup_table = @import("errol/lookup.zig").lookup_table;
    const HP = @import("errol/lookup.zig").HP;
    const math = std.math;
    const mem = std.mem;
    const assert = std.debug.assert;

    pub const FloatDecimal = struct {
        digits: []u8,
        exp: i32,
    };

    pub const RoundMode = enum {
        // Round only the fractional portion (e.g. 1234.23 has precision 2)

        Decimal,
        // Round the entire whole/fractional portion (e.g. 1.23423e3 has precision 5)

        Scientific,
    };

    /// Round a FloatDecimal as returned by errol3 to the specified fractional precision.
    /// All digits after the specified precision should be considered invalid.
    pub fn roundToPrecision(float_decimal: *FloatDecimal, precision: usize, mode: RoundMode) void {
        // The round digit refers to the index which we should look at to determine

        // whether we need to round to match the specified precision.

        var round_digit: usize = 0;

        switch (mode) {
            RoundMode.Decimal => {
                if (float_decimal.exp >= 0) {
                    round_digit = precision + @intCast(usize, float_decimal.exp);
                } else {
                    // if a small negative exp, then adjust we need to offset by the number

                    // of leading zeros that will occur.

                    const min_exp_required = @intCast(usize, -float_decimal.exp);
                    if (precision > min_exp_required) {
                        round_digit = precision - min_exp_required;
                    }
                }
            },
            RoundMode.Scientific => {
                round_digit = 1 + precision;
            },
        }

        // It suffices to look at just this digit. We don't round and propagate say 0.04999 to 0.05

        // first, and then to 0.1 in the case of a {.1} single precision.

        // Find the digit which will signify the round point and start rounding backwards.

        if (round_digit < float_decimal.digits.len and float_decimal.digits[round_digit] - '0' >= 5) {
            assert(round_digit >= 0);

            var i = round_digit;
            while (true) {
                if (i == 0) {
                    // Rounded all the way past the start. This was of the form 9.999...

                    // Slot the new digit in place and increase the exponent.

                    float_decimal.exp += 1;

                    // Re-size the buffer to use the reserved leading byte.

                    const one_before = @intToPtr([*]u8, @ptrToInt(&float_decimal.digits[0]) - 1);
                    float_decimal.digits = one_before[0 .. float_decimal.digits.len + 1];
                    float_decimal.digits[0] = '1';
                    return;
                }

                i -= 1;

                const new_value = (float_decimal.digits[i] - '0' + 1) % 10;
                float_decimal.digits[i] = new_value + '0';

                // must continue rounding until non-9

                if (new_value != 0) {
                    return;
                }
            }
        }
    }

    /// Corrected Errol3 double to ASCII conversion.
    pub fn errol3(value: f64, buffer: []u8) FloatDecimal {
        const bits = @bitCast(u64, value);
        const i = tableLowerBound(bits);
        if (i < enum3.len and enum3[i] == bits) {
            const data = enum3_data[i];
            const digits = buffer[1 .. data.str.len + 1];
            mem.copy(u8, digits, data.str);
            return FloatDecimal{
                .digits = digits,
                .exp = data.exp,
            };
        }

        // We generate digits starting at index 1. If rounding a buffer later then it may be

        // required to generate a preceding digit in some cases (9.999) in which case we use

        // the 0-index for this extra digit.

        return errol3u(value, buffer[1..]);
    }

    /// Uncorrected Errol3 double to ASCII conversion.
    fn errol3u(val: f64, buffer: []u8) FloatDecimal {
        // check if in integer or fixed range

        if (val > 9.007199254740992e15 and val < 3.40282366920938e+38) {
            return errolInt(val, buffer);
        } else if (val >= 16.0 and val < 9.007199254740992e15) {
            return errolFixed(val, buffer);
        }
        return errolSlow(val, buffer);
    }

    fn errolSlow(val: f64, buffer: []u8) FloatDecimal {
        // normalize the midpoint

        const e = math.frexp(val).exponent;
        var exp = @floatToInt(i16, @floor(307 + @intToFloat(f64, e) * 0.30103));
        if (exp < 20) {
            exp = 20;
        } else if (@intCast(usize, exp) >= lookup_table.len) {
            exp = @intCast(i16, lookup_table.len - 1);
        }

        var mid = lookup_table[@intCast(usize, exp)];
        mid = hpProd(mid, val);
        const lten = lookup_table[@intCast(usize, exp)].val;

        exp -= 307;

        var ten: f64 = 1.0;

        while (mid.val > 10.0 or (mid.val == 10.0 and mid.off >= 0.0)) {
            exp += 1;
            hpDiv10(&mid);
            ten /= 10.0;
        }

        while (mid.val < 1.0 or (mid.val == 1.0 and mid.off < 0.0)) {
            exp -= 1;
            hpMul10(&mid);
            ten *= 10.0;
        }

        // compute boundaries

        var high = HP{
            .val = mid.val,
            .off = mid.off + (fpnext(val) - val) * lten * ten / 2.0,
        };
        var low = HP{
            .val = mid.val,
            .off = mid.off + (fpprev(val) - val) * lten * ten / 2.0,
        };

        hpNormalize(&high);
        hpNormalize(&low);

        // normalized boundaries

        while (high.val > 10.0 or (high.val == 10.0 and high.off >= 0.0)) {
            exp += 1;
            hpDiv10(&high);
            hpDiv10(&low);
        }

        while (high.val < 1.0 or (high.val == 1.0 and high.off < 0.0)) {
            exp -= 1;
            hpMul10(&high);
            hpMul10(&low);
        }

        // digit generation

        var buf_index: usize = 0;
        const bound = buffer.len - 1;
        while (buf_index < bound) {
            var hdig = @floatToInt(u8, @floor(high.val));
            if ((high.val == @intToFloat(f64, hdig)) and (high.off < 0)) hdig -= 1;

            var ldig = @floatToInt(u8, @floor(low.val));
            if ((low.val == @intToFloat(f64, ldig)) and (low.off < 0)) ldig -= 1;

            if (ldig != hdig) break;

            buffer[buf_index] = hdig + '0';
            buf_index += 1;
            high.val -= @intToFloat(f64, hdig);
            low.val -= @intToFloat(f64, ldig);
            hpMul10(&high);
            hpMul10(&low);
        }

        const tmp = (high.val + low.val) / 2.0;
        var mdig = @floatToInt(u8, @floor(tmp + 0.5));
        if ((@intToFloat(f64, mdig) - tmp) == 0.5 and (mdig & 0x1) != 0) mdig -= 1;

        buffer[buf_index] = mdig + '0';
        buf_index += 1;

        return FloatDecimal{
            .digits = buffer[0..buf_index],
            .exp = exp,
        };
    }

    fn tableLowerBound(k: u64) usize {
        var i = enum3.len;
        var j: usize = 0;

        while (j < enum3.len) {
            if (enum3[j] < k) {
                j = 2 * j + 2;
            } else {
                i = j;
                j = 2 * j + 1;
            }
        }

        return i;
    }

    /// Compute the product of an HP number and a double.
    ///   @in: The HP number.
    ///   @val: The double.
    ///   &returns: The HP number.
    fn hpProd(in: HP, val: f64) HP {
        var hi: f64 = undefined;
        var lo: f64 = undefined;
        split(in.val, &hi, &lo);

        var hi2: f64 = undefined;
        var lo2: f64 = undefined;
        split(val, &hi2, &lo2);

        const p = in.val * val;
        const e = ((hi * hi2 - p) + lo * hi2 + hi * lo2) + lo * lo2;

        return HP{
            .val = p,
            .off = in.off * val + e,
        };
    }

    /// Split a double into two halves.
    ///   @val: The double.
    ///   @hi: The high bits.
    ///   @lo: The low bits.
    fn split(val: f64, hi: *f64, lo: *f64) void {
        hi.* = gethi(val);
        lo.* = val - hi.*;
    }

    fn gethi(in: f64) f64 {
        const bits = @bitCast(u64, in);
        const new_bits = bits & 0xFFFFFFFFF8000000;
        return @bitCast(f64, new_bits);
    }

    /// Normalize the number by factoring in the error.
    ///   @hp: The float pair.
    fn hpNormalize(hp: *HP) void {
        const val = hp.val;
        hp.val += hp.off;
        hp.off += val - hp.val;
    }

    /// Divide the high-precision number by ten.
    ///   @hp: The high-precision number
    fn hpDiv10(hp: *HP) void {
        var val = hp.val;

        hp.val /= 10.0;
        hp.off /= 10.0;

        val -= hp.val * 8.0;
        val -= hp.val * 2.0;

        hp.off += val / 10.0;

        hpNormalize(hp);
    }

    /// Multiply the high-precision number by ten.
    ///   @hp: The high-precision number
    fn hpMul10(hp: *HP) void {
        const val = hp.val;

        hp.val *= 10.0;
        hp.off *= 10.0;

        var off = hp.val;
        off -= val * 8.0;
        off -= val * 2.0;

        hp.off -= off;

        hpNormalize(hp);
    }

    /// Integer conversion algorithm, guaranteed correct, optimal, and best.
    ///  @val: The val.
    ///  @buf: The output buffer.
    ///  &return: The exponent.
    fn errolInt(val: f64, buffer: []u8) FloatDecimal {
        const pow19 = @as(u128, 1e19);

        assert((val > 9.007199254740992e15) and val < (3.40282366920938e38));

        var mid = @floatToInt(u128, val);
        var low: u128 = mid - fpeint((fpnext(val) - val) / 2.0);
        var high: u128 = mid + fpeint((val - fpprev(val)) / 2.0);

        if (@bitCast(u64, val) & 0x1 != 0) {
            high -= 1;
        } else {
            low -= 1;
        }

        var l64 = @intCast(u64, low % pow19);
        const lf = @intCast(u64, (low / pow19) % pow19);

        var h64 = @intCast(u64, high % pow19);
        const hf = @intCast(u64, (high / pow19) % pow19);

        if (lf != hf) {
            l64 = lf;
            h64 = hf;
            mid = mid / (pow19 / 10);
        }

        var mi: i32 = mismatch10(l64, h64);
        var x: u64 = 1;
        {
            var i: i32 = @boolToInt(lf == hf);
            while (i < mi) : (i += 1) {
                x *= 10;
            }
        }
        const m64 = @truncate(u64, @divTrunc(mid, x));

        if (lf != hf) mi += 19;

        var buf_index = u64toa(m64, buffer) - 1;

        if (mi != 0) {
            const round_up = buffer[buf_index] >= '5';
            if (buf_index == 0 or (round_up and buffer[buf_index - 1] == '9')) return errolSlow(val, buffer);
            buffer[buf_index - 1] += @boolToInt(round_up);
        } else {
            buf_index += 1;
        }

        return FloatDecimal{
            .digits = buffer[0..buf_index],
            .exp = @intCast(i32, buf_index) + mi,
        };
    }

    /// Fixed point conversion algorithm, guaranteed correct, optimal, and best.
    ///  @val: The val.
    ///  @buf: The output buffer.
    ///  &return: The exponent.
    fn errolFixed(val: f64, buffer: []u8) FloatDecimal {
        assert((val >= 16.0) and (val < 9.007199254740992e15));

        const u = @floatToInt(u64, val);
        const n = @intToFloat(f64, u);

        var mid = val - n;
        var lo = ((fpprev(val) - n) + mid) / 2.0;
        var hi = ((fpnext(val) - n) + mid) / 2.0;

        var buf_index = u64toa(u, buffer);
        var exp = @intCast(i32, buf_index);
        var j = buf_index;
        buffer[j] = 0;

        if (mid != 0.0) {
            while (mid != 0.0) {
                lo *= 10.0;
                const ldig = @floatToInt(i32, lo);
                lo -= @intToFloat(f64, ldig);

                mid *= 10.0;
                const mdig = @floatToInt(i32, mid);
                mid -= @intToFloat(f64, mdig);

                hi *= 10.0;
                const hdig = @floatToInt(i32, hi);
                hi -= @intToFloat(f64, hdig);

                buffer[j] = @intCast(u8, mdig + '0');
                j += 1;

                if (hdig != ldig or j > 50) break;
            }

            if (mid > 0.5) {
                buffer[j - 1] += 1;
            } else if ((mid == 0.5) and (buffer[j - 1] & 0x1) != 0) {
                buffer[j - 1] += 1;
            }
        } else {
            while (buffer[j - 1] == '0') {
                buffer[j - 1] = 0;
                j -= 1;
            }
        }

        buffer[j] = 0;

        return FloatDecimal{
            .digits = buffer[0..j],
            .exp = exp,
        };
    }

    fn fpnext(val: f64) f64 {
        return @bitCast(f64, @bitCast(u64, val) +% 1);
    }

    fn fpprev(val: f64) f64 {
        return @bitCast(f64, @bitCast(u64, val) -% 1);
    }

    pub const c_digits_lut = [_]u8{
        '0', '0', '0', '1', '0', '2', '0', '3', '0', '4', '0', '5', '0', '6',
        '0', '7', '0', '8', '0', '9', '1', '0', '1', '1', '1', '2', '1', '3',
        '1', '4', '1', '5', '1', '6', '1', '7', '1', '8', '1', '9', '2', '0',
        '2', '1', '2', '2', '2', '3', '2', '4', '2', '5', '2', '6', '2', '7',
        '2', '8', '2', '9', '3', '0', '3', '1', '3', '2', '3', '3', '3', '4',
        '3', '5', '3', '6', '3', '7', '3', '8', '3', '9', '4', '0', '4', '1',
        '4', '2', '4', '3', '4', '4', '4', '5', '4', '6', '4', '7', '4', '8',
        '4', '9', '5', '0', '5', '1', '5', '2', '5', '3', '5', '4', '5', '5',
        '5', '6', '5', '7', '5', '8', '5', '9', '6', '0', '6', '1', '6', '2',
        '6', '3', '6', '4', '6', '5', '6', '6', '6', '7', '6', '8', '6', '9',
        '7', '0', '7', '1', '7', '2', '7', '3', '7', '4', '7', '5', '7', '6',
        '7', '7', '7', '8', '7', '9', '8', '0', '8', '1', '8', '2', '8', '3',
        '8', '4', '8', '5', '8', '6', '8', '7', '8', '8', '8', '9', '9', '0',
        '9', '1', '9', '2', '9', '3', '9', '4', '9', '5', '9', '6', '9', '7',
        '9', '8', '9', '9',
    };

    fn u64toa(value_param: u64, buffer: []u8) usize {
        var value = value_param;
        const kTen8: u64 = 100000000;
        const kTen9: u64 = kTen8 * 10;
        const kTen10: u64 = kTen8 * 100;
        const kTen11: u64 = kTen8 * 1000;
        const kTen12: u64 = kTen8 * 10000;
        const kTen13: u64 = kTen8 * 100000;
        const kTen14: u64 = kTen8 * 1000000;
        const kTen15: u64 = kTen8 * 10000000;
        const kTen16: u64 = kTen8 * kTen8;

        var buf_index: usize = 0;

        if (value < kTen8) {
            const v = @intCast(u32, value);
            if (v < 10000) {
                const d1: u32 = (v / 100) << 1;
                const d2: u32 = (v % 100) << 1;

                if (v >= 1000) {
                    buffer[buf_index] = c_digits_lut[d1];
                    buf_index += 1;
                }
                if (v >= 100) {
                    buffer[buf_index] = c_digits_lut[d1 + 1];
                    buf_index += 1;
                }
                if (v >= 10) {
                    buffer[buf_index] = c_digits_lut[d2];
                    buf_index += 1;
                }
                buffer[buf_index] = c_digits_lut[d2 + 1];
                buf_index += 1;
            } else {
                // value = bbbbcccc

                const b: u32 = v / 10000;
                const c: u32 = v % 10000;

                const d1: u32 = (b / 100) << 1;
                const d2: u32 = (b % 100) << 1;

                const d3: u32 = (c / 100) << 1;
                const d4: u32 = (c % 100) << 1;

                if (value >= 10000000) {
                    buffer[buf_index] = c_digits_lut[d1];
                    buf_index += 1;
                }
                if (value >= 1000000) {
                    buffer[buf_index] = c_digits_lut[d1 + 1];
                    buf_index += 1;
                }
                if (value >= 100000) {
                    buffer[buf_index] = c_digits_lut[d2];
                    buf_index += 1;
                }
                buffer[buf_index] = c_digits_lut[d2 + 1];
                buf_index += 1;

                buffer[buf_index] = c_digits_lut[d3];
                buf_index += 1;
                buffer[buf_index] = c_digits_lut[d3 + 1];
                buf_index += 1;
                buffer[buf_index] = c_digits_lut[d4];
                buf_index += 1;
                buffer[buf_index] = c_digits_lut[d4 + 1];
                buf_index += 1;
            }
        } else if (value < kTen16) {
            const v0: u32 = @intCast(u32, value / kTen8);
            const v1: u32 = @intCast(u32, value % kTen8);

            const b0: u32 = v0 / 10000;
            const c0: u32 = v0 % 10000;

            const d1: u32 = (b0 / 100) << 1;
            const d2: u32 = (b0 % 100) << 1;

            const d3: u32 = (c0 / 100) << 1;
            const d4: u32 = (c0 % 100) << 1;

            const b1: u32 = v1 / 10000;
            const c1: u32 = v1 % 10000;

            const d5: u32 = (b1 / 100) << 1;
            const d6: u32 = (b1 % 100) << 1;

            const d7: u32 = (c1 / 100) << 1;
            const d8: u32 = (c1 % 100) << 1;

            if (value >= kTen15) {
                buffer[buf_index] = c_digits_lut[d1];
                buf_index += 1;
            }
            if (value >= kTen14) {
                buffer[buf_index] = c_digits_lut[d1 + 1];
                buf_index += 1;
            }
            if (value >= kTen13) {
                buffer[buf_index] = c_digits_lut[d2];
                buf_index += 1;
            }
            if (value >= kTen12) {
                buffer[buf_index] = c_digits_lut[d2 + 1];
                buf_index += 1;
            }
            if (value >= kTen11) {
                buffer[buf_index] = c_digits_lut[d3];
                buf_index += 1;
            }
            if (value >= kTen10) {
                buffer[buf_index] = c_digits_lut[d3 + 1];
                buf_index += 1;
            }
            if (value >= kTen9) {
                buffer[buf_index] = c_digits_lut[d4];
                buf_index += 1;
            }
            if (value >= kTen8) {
                buffer[buf_index] = c_digits_lut[d4 + 1];
                buf_index += 1;
            }

            buffer[buf_index] = c_digits_lut[d5];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d5 + 1];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d6];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d6 + 1];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d7];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d7 + 1];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d8];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d8 + 1];
            buf_index += 1;
        } else {
            const a = @intCast(u32, value / kTen16); // 1 to 1844

            value %= kTen16;

            if (a < 10) {
                buffer[buf_index] = '0' + @intCast(u8, a);
                buf_index += 1;
            } else if (a < 100) {
                const i: u32 = a << 1;
                buffer[buf_index] = c_digits_lut[i];
                buf_index += 1;
                buffer[buf_index] = c_digits_lut[i + 1];
                buf_index += 1;
            } else if (a < 1000) {
                buffer[buf_index] = '0' + @intCast(u8, a / 100);
                buf_index += 1;

                const i: u32 = (a % 100) << 1;
                buffer[buf_index] = c_digits_lut[i];
                buf_index += 1;
                buffer[buf_index] = c_digits_lut[i + 1];
                buf_index += 1;
            } else {
                const i: u32 = (a / 100) << 1;
                const j: u32 = (a % 100) << 1;
                buffer[buf_index] = c_digits_lut[i];
                buf_index += 1;
                buffer[buf_index] = c_digits_lut[i + 1];
                buf_index += 1;
                buffer[buf_index] = c_digits_lut[j];
                buf_index += 1;
                buffer[buf_index] = c_digits_lut[j + 1];
                buf_index += 1;
            }

            const v0 = @intCast(u32, value / kTen8);
            const v1 = @intCast(u32, value % kTen8);

            const b0: u32 = v0 / 10000;
            const c0: u32 = v0 % 10000;

            const d1: u32 = (b0 / 100) << 1;
            const d2: u32 = (b0 % 100) << 1;

            const d3: u32 = (c0 / 100) << 1;
            const d4: u32 = (c0 % 100) << 1;

            const b1: u32 = v1 / 10000;
            const c1: u32 = v1 % 10000;

            const d5: u32 = (b1 / 100) << 1;
            const d6: u32 = (b1 % 100) << 1;

            const d7: u32 = (c1 / 100) << 1;
            const d8: u32 = (c1 % 100) << 1;

            buffer[buf_index] = c_digits_lut[d1];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d1 + 1];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d2];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d2 + 1];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d3];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d3 + 1];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d4];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d4 + 1];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d5];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d5 + 1];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d6];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d6 + 1];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d7];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d7 + 1];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d8];
            buf_index += 1;
            buffer[buf_index] = c_digits_lut[d8 + 1];
            buf_index += 1;
        }

        return buf_index;
    }

    fn fpeint(from: f64) u128 {
        const bits = @bitCast(u64, from);
        assert((bits & ((1 << 52) - 1)) == 0);

        return @as(u128, 1) << @truncate(u7, (bits >> 52) -% 1023);
    }

    /// Given two different integers with the same length in terms of the number
    /// of decimal digits, index the digits from the right-most position starting
    /// from zero, find the first index where the digits in the two integers
    /// divergent starting from the highest index.
    ///   @a: Integer a.
    ///   @b: Integer b.
    ///   &returns: An index within [0, 19).
    fn mismatch10(a: u64, b: u64) i32 {
        const pow10 = 10000000000;
        const af = a / pow10;
        const bf = b / pow10;

        var i: i32 = 0;
        var a_copy = a;
        var b_copy = b;

        if (af != bf) {
            i = 10;
            a_copy = af;
            b_copy = bf;
        }

        while (true) : (i += 1) {
            a_copy /= 10;
            b_copy /= 10;

            if (a_copy == b_copy) return i;
        }
    }