A dead simple blockchain written in Dart in 93 lines

blockchain.dart

import 'dart:collection';
import 'dart:convert' as convert;
import 'package:crypto/crypto.dart' as crypto;

import 'package:flutter_test/flutter_test.dart';

main() {
  testWidgets('Create, add, pass validate, tamper, fail validation', (_) async {
    final blockchain = Blockchain();
    expect(blockchain.isValid, isTrue);

    blockchain.addBlock("First");
    blockchain.addBlock("Second");
    expect(blockchain.isValid, isTrue);

    blockchain._chain.removeAt(1);
    expect(blockchain.isValid, isFalse);
  });
}

/// A dead simple blockchain sytem where each block carries a String payload
class Blockchain {
  final List<Block> _chain = [Block.genesis()];

  /// Add a block to the top of the chain
  String addBlock(String message) {
    final block = Block(_chain.last.hash, message);
    _chain.add(block);
    return block.hash;
  }

  /// Compute if the chain is valid
  bool get isValid {
    /// The chain is valid if it only has a genesis block
    if (_chain.length == 1) return true;

    bool isValid = true;

    _chain.pairwise((a, b) {
      /// Given two adjacent blocks, the first block's computed hash value should
      /// equal the stored _previousHash value on the second block. This is the link in the chain.
      if (b._previousHash != a.hash) {
        isValid = false;
      }
    });

    return isValid;
  }

  /// Get the current raw chain.
  List<Block> get chain => UnmodifiableListView<Block>(_chain);
}

/// The entry must contain the previous block hash and its current value.
/// We can then compute it's hash on the fly.
class Block {
  Block(this._previousHash, this.payload) : this.forgeDate = DateTime.now();

  /// A special constructor for the genesis block
  Block.genesis()
      : this._previousHash = "genesis",
        this.payload = "genesis",
        this.forgeDate = DateTime.now();

  /// The value to store in the block. In Bitcoin this would be a list of transactions,
  /// for our purposes it will just be a string.
  final String payload;

  final DateTime forgeDate;

  final String _previousHash;

  /// To maintain chain integrity we roll the previous block hash into this block's hashed data.
  String get hash => "$payload@$forgeDate via $_previousHash".sha256;
}

extension ListExtensions<E> on List<E> {
  /// Like [forEach] but comparing two adjacent entries
  void pairwise(void Function(E a, E b) f) {
    for (int i = 0; i < this.length - 1; i++) {
      f(this[i], this[i + 1]);
    }
  }
}

extension on String {
  /// The UTF8 data list of this String
  List<int> get utf8 => convert.utf8.encode(this);

  /// The SHA256 hash of this String
  String get sha256 => crypto.sha256.convert(this.utf8).toString();
}