RSpec::Checksum::Matchers: Check if a String looks like a digest produced a the given hash algorithm. Also see https://github.com/sshaw/has_checksum

rspec-checksum-matchers.rb

# RSpec::Checksum::Matchers Check if a String looks like a checksum produced by the given algorithm.
# https://gist.github.com/sshaw/df14f6f89860b2dbcfd2
#
# Copyright (c) 2016 Skye Shaw. Released under the MIT License (https://opensource.org/licenses/MIT)
#
require "rspec/expectations"

RSpec::Matchers.define :be_a_checksum do
  regex = /\A[0-9a-f]{32,128}\z/i
  chars = {
    :md2  => 32,
    :md4  => 32,
    :md5  => 32,
    :sha1 => 40,
    :sha224 => 56,
    :sha256 => 64,
    :sha384 => 96,
    :sha512 => 128
  }

  match do |checksum|
    checksum = String(checksum)
    return checksum.bytesize == chars[@function] / 2 if @use_bytes # 2 hex chars = 1 byte
    matched = !!(checksum =~ regex)
    return matched unless @function && matched
    checksum.size == chars[@function]
  end

  chain :produced_by do |name|
    @function = name.to_s.downcase.to_sym
    raise ArgumentError, "unknown algorithm given to be_a_checksum.produced_by: #@function" unless chars.include?(@function)
  end

  chain :of_bytes do
    @use_bytes = true
  end
end


if File.basename($0) == "rspec"
  require "digest"

  RSpec.describe "Checksum expectations" do
    describe "#be_a_checksum" do
      it "matches a checksum" do
        expect(Digest::MD5.hexdigest("a")).to be_a_checksum
        expect("a").to_not be_a_checksum
      end
    end

    describe "#produced_by" do
      it "matches a MD5" do
        expect(Digest::MD5.hexdigest("a")).to be_a_checksum.produced_by(:md5)
      end

      it "matches SHA1" do
        expect(Digest::SHA1.hexdigest("a")).to be_a_checksum.produced_by(:sha1)
      end

      it "matches SHA256" do
        expect(Digest::SHA256.hexdigest("a")).to be_a_checksum.produced_by(:sha256)
      end

      it "raises an exception when the algorithm is not known" do
        expect { expect("a").to be_a_checksum.produced_by(:foo) }.to raise_exception(/unknown algorithm/)
      end

      it "does not match when the wrong algorithm is given" do
        expect(Digest::SHA256.hexdigest("a")).to_not be_a_checksum.produced_by(:md5)
      end
    end

    describe "#of_bytes" do
      it "matches a bytes string" do
        md5 = Digest::MD5.new
        md5 << "a"
        expect(md5.digest).to be_a_checksum.produced_by(:md5).of_bytes
      end
    end
  end
end

Just what I was looking for, thank you!

@sshaw Love this! There's no license on this, is this MIT like the rest of your github repos? Can you add one so I can use this?

@shawn42, yes it's MIT. I added this to the top of the file.

@sshaw thanks so much for this. Came in handy.