Manual (slightly faster, but unsafe) implementation of Atoi for converting strings into integers

atoi.go

package main

import (
	"fmt"
	"math"
)

func main() {
  n := "12345"
  v := 0.0

  // Convert the string byte by byte
  // We're not doing any error checking for pure speed so be very careful
  // pre-validating your input.
  // nb. If you wrap this in a function call you'll lose efficiency
  //
  // Benchmarked against strconv.Atoi in 3s:
  //
  // BenchmarkAtoi-12      	469316667	         7.51 ns/op
  // BenchmarkManual-12    	613654023	         5.85 ns/op
  //
  for i := 0; i < len(n); i++ {
      v += float64(n[i] - 48) * math.Pow10(len(n)-i-1)
  }
  
  fmt.Printf("%s -> %d\n",n,int(v))
}

// cpu: AMD Ryzen 5 4500U with Radeon Graphics         
// BenchmarkFastAtoi
// BenchmarkFastAtoi-6   	443713263	         2.705 ns/op
//
func FastAtoi(s string) int {
	var val int

	for _, c := range s {
		val = val*10 + int(c-'0')
	}

	return val
}