shad-go/varfmt/fmt.go

//go:build !solution

package varfmt

import (
	"fmt"
	"strconv"
	"strings"
	"unicode/utf8"
)

// fmt.Sprintf does 1 alloc
// this does 3-4 allocations and is a bit slower than fmt.Sprintf
// can be further optimized, by casting args slice
// to string without allocating new space (somehow)
func Sprintf(format string, args ...any) string {
	// init output strings.Builder and num rune buffer
	// cant use strings.Builder for storing num since
	// builder can't clear its contents without reallocating
	output, num := strings.Builder{}, make([]rune, 0, 2) // first alloc here
	// convert args slice to strArgs string slice
	strArgs, maxLen := make([]string, len(args), cap(args)), 0 // second alloc here
	for i, arg := range args {
		strArgs[i] = fmt.Sprint(arg) // third and other allocs happen here
		// could be further optimized
		if len(strArgs[i]) > maxLen {
			maxLen = len(strArgs[i])
		}
	}
	// allocate maximum possible length of the resulting string
	output.Grow(len(format) + len(args)*maxLen) // third alloc here
	start, argIndex := 0, 0
	// decode runes one by one
	for start < len(format) {
		r, n := utf8.DecodeRuneInString(format[start:])
		start += n
		// if specifier is found, start forming index
		if r == '{' {
			numLength := 0
			for {
				r, n = utf8.DecodeRuneInString(format[start:])
				start += n
				if r == '}' {
					break
				}
				numLength++
				// append new rune if the number is bigger
				// than the current buffer
				if numLength >= len(num) {
					num = append(num, r) // further allocs happen here
				} else {
					num[numLength] = r
				}
			}
			var index int
			// if specifier is empty get arg at argIndex
			// otherwise get arg at the specified index from num
			if len(num) == 0 {
				index = argIndex
			} else {
				var err error
				index, err = strconv.Atoi(string(num))
				if err != nil {
					panic(err)
				}
			}
			output.WriteString(strArgs[index])
			argIndex++
			// set len of num to 0, but keep capacity
			num = num[:0]
			continue
		}
		output.WriteRune(r)
	}
	return output.String()
}