rust 22 lines · 5 steps

Streaming a SHA-256 hash of a file in Rust

Compute a file's SHA-256 digest by reading it in fixed-size chunks so memory stays constant regardless of file size.

Explained by highlit
Intermediate hashing buffered-io streaming error-handling
1use std::fs::File;
2use std::io::{self, Read};
3use std::path::Path;
4 
5use sha2::{Digest, Sha256};
6 
7pub fn sha256_file(path: impl AsRef<Path>) -> io::Result<String> {
8 let mut file = File::open(path)?;
9 let mut hasher = Sha256::new();
10 let mut buffer = [0u8; 64 * 1024];
11 
12 loop {
13 let read = file.read(&mut buffer)?;
14 if read == 0 {
15 break;
16 }
17 hasher.update(&buffer[..read]);
18 }
19 
20 let digest = hasher.finalize();
21 Ok(hex::encode(digest))
22}
01 / 01
STEP 01

Walkthrough

Space play ←→ step click any line
Three takeaways
  1. 1Reading a file in a fixed buffer keeps memory flat no matter how large the file grows.
  2. 2Hashers accept data incrementally, so you never need the whole input in memory at once.
  3. 3Using `impl AsRef<Path>` lets callers pass strings, `PathBuf`, or `Path` interchangeably.

Related explainers

rust 
use serde::Deserialize;
 
#[derive(Debug, Deserialize)]
#[serde(untagged)]

Parsing flexible JSON shapes with serde

deserialization enums json
Intermediate 6 steps
ruby 
require "shellwords"
require "open3"
 
module Backup

Building safe shell commands in Ruby

shell-out subprocess command-injection
Intermediate 7 steps
rust 
use std::cmp::{Ordering, Reverse};
use std::collections::BinaryHeap;
use std::fs::File;
use std::io::{self, BufRead, BufReader, BufWriter, Lines, Write};

K-way merge of sorted logs in Rust

binary-heap k-way-merge streaming-io
Intermediate 8 steps
rust 
use axum::{
    extract::{Path, State},
    response::sse::{Event, KeepAlive, Sse},
};

Streaming import progress with SSE in Axum

server-sent-events streams watch-channel
Advanced 7 steps
ruby 
class LogAggregator
  BUCKET_FORMAT = "%Y-%m-%dT%H:%M"
 
  def initialize(entries)

Bucketing log entries by the minute in Ruby

aggregation hashing enumerable
Intermediate 5 steps
rust 
use std::f64::consts::PI;
 
#[derive(Debug, Clone, Copy)]
pub struct LatLng {

Building geographic bounding boxes in Rust

geospatial value-types option
Intermediate 7 steps

Share this explainer

Here's the card — post it anywhere.

Streaming a SHA-256 hash of a file in Rust — share card
Made with highlit — turn any snippet into a walkthrough like this in about a minute.
Explain your code