src/internal/strconv/itoa.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package strconv

import "math/bits"

// FormatUint returns the string representation of i in the given base,
// for 2 <= base <= 36. The result uses the lower-case letters 'a' to 'z'
// for digit values >= 10.
func FormatUint(i uint64, base int) string {
	if base == 10 {
		if i < nSmalls {
			return small(int(i))
		}
		var a [24]byte
		nd := numDigits(i)
		formatBase10(a[:nd], i)
		return string(a[:nd])
	}
	_, s := formatBits(nil, i, base, false, false)
	return s
}

// FormatInt returns the string representation of i in the given base,
// for 2 <= base <= 36. The result uses the lower-case letters 'a' to 'z'
// for digit values >= 10.
func FormatInt(i int64, base int) string {
	if base == 10 {
		if 0 <= i && i < nSmalls {
			return small(int(i))
		}
		var a [24]byte
		u := uint64(i)
		if i < 0 {
			u = -u
		}
		nd := numDigits(u)
		formatBase10(a[1:1+nd], u)
		if i < 0 {
			a[0] = '-'
			return string(a[:1+nd])
		}
		return string(a[1 : 1+nd])
	}
	_, s := formatBits(nil, uint64(i), base, i < 0, false)
	return s
}

// Itoa is equivalent to [FormatInt](int64(i), 10).
func Itoa(i int) string {
	return FormatInt(int64(i), 10)
}

// AppendInt appends the string form of the integer i,
// as generated by [FormatInt], to dst and returns the extended buffer.
func AppendInt(dst []byte, i int64, base int) []byte {
	u := uint64(i)
	if i < 0 {
		dst = append(dst, '-')
		u = -u
	}
	return AppendUint(dst, u, base)
}

// AppendUint appends the string form of the unsigned integer i,
// as generated by [FormatUint], to dst and returns the extended buffer.
func AppendUint(dst []byte, i uint64, base int) []byte {
	if base == 10 {
		if i < nSmalls {
			return append(dst, small(int(i))...)
		}
		var a [24]byte
		nd := numDigits(i)
		formatBase10(a[:nd], i)
		return append(dst, a[:nd]...)
	}
	dst, _ = formatBits(dst, i, base, false, true)
	return dst
}

const digits = "0123456789abcdefghijklmnopqrstuvwxyz"

// formatBits computes the string representation of u in the given base.
// If neg is set, u is treated as negative int64 value. If append_ is
// set, the string is appended to dst and the resulting byte slice is
// returned as the first result value; otherwise the string is returned
// as the second result value.
// The caller is expected to have handled base 10 separately for speed.
func formatBits(dst []byte, u uint64, base int, neg, append_ bool) (d []byte, s string) {
	if base < 2 || base == 10 || base > len(digits) {
		panic("strconv: illegal AppendInt/FormatInt base")
	}
	// 2 <= base && base <= len(digits)

	var a [64 + 1]byte // +1 for sign of 64bit value in base 2
	i := len(a)
	if neg {
		u = -u
	}

	// convert bits
	// We use uint values where we can because those will
	// fit into a single register even on a 32bit machine.
	if isPowerOfTwo(base) {
		// Use shifts and masks instead of / and %.
		shift := uint(bits.TrailingZeros(uint(base)))
		b := uint64(base)
		m := uint(base) - 1 // == 1<<shift - 1
		for u >= b {
			i--
			a[i] = digits[uint(u)&m]
			u >>= shift
		}
		// u < base
		i--
		a[i] = digits[uint(u)]
	} else {
		// general case
		b := uint64(base)
		for u >= b {
			i--
			// Avoid using r = a%b in addition to q = a/b
			// since 64bit division and modulo operations
			// are calculated by runtime functions on 32bit machines.
			q := u / b
			a[i] = digits[uint(u-q*b)]
			u = q
		}
		// u < base
		i--
		a[i] = digits[uint(u)]
	}

	// add sign, if any
	if neg {
		i--
		a[i] = '-'
	}

	if append_ {
		d = append(dst, a[i:]...)
		return
	}
	s = string(a[i:])
	return
}

func isPowerOfTwo(x int) bool {
	return x&(x-1) == 0
}

const nSmalls = 100

// smalls is the formatting of 00..99 concatenated.
// It is then padded out with 56 x's to 256 bytes,
// so that smalls[x&0xFF] has no bounds check.
const smalls = "00010203040506070809" +
	"10111213141516171819" +
	"20212223242526272829" +
	"30313233343536373839" +
	"40414243444546474849" +
	"50515253545556575859" +
	"60616263646566676869" +
	"70717273747576777879" +
	"80818283848586878889" +
	"90919293949596979899"

// small returns the string for an i with 0 <= i < nSmalls.
func small(i int) string {
	if i < 10 {
		return digits[i : i+1]
	}
	return smalls[i*2 : i*2+2]
}

// RuntimeFormatBase10 formats u into the tail of a
// and returns the offset to the first byte written to a.
// It is only for use by package runtime.
// Other packages should use AppendUint.
func RuntimeFormatBase10(a []byte, u uint64) int {
	// Note: numDigits requires an argument ≥ 1.
	// The |1 changes 0 to 1 without adding an extra digit
	// to any other value.
	i := len(a) - numDigits(u|1)
	formatBase10(a[i:], u)
	return i
}

// formatBase10 formats the decimal representation of u into a.
// The caller is responsible for ensuring that a is big enough to hold u.
// If a is too big, leading zeros will be filled in as needed.
func formatBase10(a []byte, u uint64) {
	nd := len(a)
	for nd >= 8 {
		// Format last 8 digits (4 pairs).
		x3210 := uint32(u % 1e8)
		u /= 1e8
		x32, x10 := x3210/1e4, x3210%1e4
		x1, x0 := (x10/100)*2, (x10%100)*2
		x3, x2 := (x32/100)*2, (x32%100)*2
		a[nd-1], a[nd-2] = smalls[x0+1], smalls[x0]
		a[nd-3], a[nd-4] = smalls[x1+1], smalls[x1]
		a[nd-5], a[nd-6] = smalls[x2+1], smalls[x2]
		a[nd-7], a[nd-8] = smalls[x3+1], smalls[x3]
		nd -= 8
	}

	x := uint32(u)
	if nd >= 4 {
		// Format last 4 digits (2 pairs).
		x10 := x % 1e4
		x /= 1e4
		x1, x0 := (x10/100)*2, (x10%100)*2
		a[nd-1], a[nd-2] = smalls[x0+1], smalls[x0]
		a[nd-3], a[nd-4] = smalls[x1+1], smalls[x1]
		nd -= 4
	}
	if nd >= 2 {
		// Format last 2 digits.
		x0 := (x % 1e2) * 2
		x /= 1e2
		a[nd-1], a[nd-2] = smalls[x0+1], smalls[x0]
		nd -= 2
	}
	if nd > 0 {
		// Format final digit.
		a[0] = byte('0' + x)
	}
}