shannon entropy calculation and script type matching

packages/caravan-health/src/fingerprint.test.ts

@@ -0,0 +1,21 @@
+import { Network } from "@caravan/bitcoin";
+import { amountFingerprints, scriptTypeFingerprints } from "./fingerprint";
+
+describe("Fingerprint", () => {
+  it("can match by hrp", () => {
+    const result = scriptTypeFingerprints(
+      "P2WSH",
+      [
+        "bc1qzcwk6mnfmdt6rx0qp0dwfll4gj8540yamdka7r",
+        "3EutoNctCnVBpVz8xcAvHBSNxSbmgWxZmV",
+      ],
+      Network.MAINNET,
+    );
+    expect(result).toEqual([true, false]);
+  });
+
+  it("can measure amount entropy", () => {
+    const result = amountFingerprints(["40.8199902", "1.18000000"]);
+    expect(result).toEqual([true, false]);
+  });
+});

packages/caravan-health/src/fingerprint.ts

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+import { getAddressType, Network } from "@caravan/bitcoin";
+import { ScriptType } from "@caravan/fees";
+
+export function scriptTypeFingerprints(
+  inputScriptType: ScriptType,
+  outputAddresses: string[],
+  network: Network,
+): boolean[] {
+  return outputAddresses.map((address) => {
+    const addressType = getAddressType(address, network);
+    return addressType === inputScriptType;
+  });
+}
+
+function log2(x: number): number {
+  return Math.log(x) / Math.LN2;
+}
+
+function shannonEntropy(str: string): number {
+  const counts: Record<string, number> = {};
+  for (const char of str) {
+    counts[char] = (counts[char] || 0) + 1;
+  }
+  const total = str.length;
+  return Object.values(counts).reduce((entropy, count) => {
+    const probability = count / total;
+    return entropy - probability * log2(probability);
+  }, 0);
+}
+
+export function amountFingerprints(amounts: string[]): boolean[] {
+  if (amounts.length === 0) return [];
+
+  // Right pad to 8 decimals
+  const paddedRight = amounts.map((a) => {
+    const [whole, frac = ""] = a.split(".");
+    return whole + "." + frac.padEnd(8, "0");
+  });
+
+  // Remove dot for uniformity, then left pad to max length
+  const noDot = paddedRight.map((a) => a.replace(".", ""));
+  const maxLen = Math.max(...noDot.map((a) => a.length));
+  const paddedAmounts = noDot.map((a) => a.padStart(maxLen, "0"));
+
+  // Compute entropy for each padded amount
+  const entropies = paddedAmounts.map((a) => shannonEntropy(a));
+  const maxEntropy = Math.max(...entropies);
+
+  // Mark true if this output has the maximum entropy
+  return entropies.map((e) => e === maxEntropy);
+}