1 files changed, 73 insertions, 0 deletions
diff --git a/pbt/ts/src/gen.ts b/pbt/ts/src/gen.ts
new file mode 100644
index 0000000..531a118
--- /dev/null
+++ b/pbt/ts/src/gen.ts
@@ -0,0 +1,73 @@
+import {Option, none, some} from 'fp-ts/lib/Option.js';
+import Prando from 'prando';
+
+// a shrinkable generator needs to maintain some state,
+// namely the sequence of generated values in order to be
+// able to backtrack and shrink.
+
+type GenMode = 'gen' | 'shrink';
+
+export interface Gen<A> {
+  generate(): A;
+
+  generateN(n: number): A[];
+}
+
+export interface GenBytes {
+  generate(n: number): number[];
+}
+
+export class BytesGen implements GenBytes {
+  prng: Prando;
+  bytes: number[][]; // the internal sequence of generated values
+
+  constructor(prng: Prando) {
+    this.prng = prng;
+    this.bytes = [];
+  }
+
+  generate(n: number): number[] {
+    if (n <= 0) {
+      throw new Error('n must be greater than 0');
+    }
+    return this.generateBytes(n);
+  }
+
+  generateBytes(n: number): number[] {
+    const gens = [];
+    for (let i = 0; i < n; i++) {
+      gens.push(this.prng.nextInt(0, 255));
+    }
+    this.bytes.push([...gens]);
+    return gens;
+  }
+
+  replay(): GenBytes {
+    return new ReplayBytes(this.bytes);
+  }
+}
+
+export class ReplayBytes implements GenBytes {
+  bytes: number[][]; // the internal sequence of generated values
+  index: number = 0; // the current index in the sequence of generated values
+
+  constructor(bytes: number[][]) {
+    this.bytes = bytes;
+    this.index = 0;
+  }
+
+  generate(n: number): number[] {
+    if (this.index >= this.bytes.length) {
+      throw new Error('No more bytes to shrink');
+    }
+
+    const chunk = [...this.bytes[this.index++]];
+
+    if (chunk.length < n) {
+      const zeros = new Array(n - chunk.length).fill(0);
+      chunk.unshift(...zeros);
+    }
+
+    return chunk;
+  }
+}