package util

import (
	"crypto/sha256"
	"encoding/hex"
	"io"
	"os"
)

// SHA256File returns the SHA-256 digest of the contents of the file at path,
// written as lowercase hexadecimal, in the same form as the sha256sum command
// prints. The file is copied through the hash in chunks rather than read into
// memory, so its size does not matter.
//
// The error is whatever went wrong opening or reading the file, passed along
// unchanged so a caller can test it with os.IsNotExist and the like.
func SHA256File(path string) (string, error) {
	file, err := os.Open(path)
	if err != nil {
		return "", err
	}
	defer file.Close()

	digest := sha256.New()
	if _, err := io.Copy(digest, file); err != nil {
		return "", err
	}

	return hex.EncodeToString(digest.Sum(nil)), nil
}