Improved version of https://gist.github.com/samuell/5591369 , removing the pointer dereference, on tip from @dgryski : https://twitter.com/dgryski/status/471045782117056512

gc_rp_ptr_woderef.go

package main

import (
	"bufio"
	"fmt"
	"log"
	"os"
)

func main() {
	var at, gc int
	counters := [256]int{
		'A': 0,
		'T': 0,
		'G': 0,
		'C': 0,
	}
	file, err := os.Open("Homo_sapiens.GRCh37.67.dna_rm.chromosome.Y.fa")
	if err != nil {
		log.Fatal(err)
	}
	scan := bufio.NewScanner(file)
	for scan.Scan() {
		line := scan.Bytes()
		if len(line) == 0 || line[0] == '>' {
			continue
		}
		for _, c := range line {
			counters[c]++
		}
	}
	at = counters['A'] + counters['T']
	gc = counters['G'] + counters['C']
	gcFraction := float32(gc) / float32(at+gc)
	fmt.Println(gcFraction * 100)
}