Test for stackoverflow question http://bit.ly/f3Tn0f. Runs in nodejs.

uuid-truncated-hash-collision-count.js

var uuid = require('node-uuid'),     // npm install node-uuid
    crypto = require('crypto');


var UUID_NUM = 1000000;


var uuid_table = {};


var methods = {
    "sha1_take_first_20": {
        descr: "Take first 20 characters of a SHA1 hex string",
        fn: function (uuid)
        {
            var sha1 = crypto.createHash("sha1");
            sha1.update(uuid);
            return sha1.digest("hex").substring(0,20);
        },
        hashes: {},
        collision_count: 0
    }
}


var i, id;

console.log("Generating", UUID_NUM, "UUIDs...");
for (i = 0; i < UUID_NUM; i++) {
    id = uuid();
    if (uuid_table[id]) {
        console.log("Ignoring duplicated ID");
        i--;
    } else {
        uuid_table[id] = true;
    }
}
console.log("Done");


var method, hash;

for (method in methods) {
    console.log("Testing method", method, "...");

    for (id in uuid_table) {
        hash = methods[method].fn(id);
        if (methods[method].hashes[hash]) {
            methods[method].collision_count++;
            //console.log("collision! hash", hash, ", uuid", id);
        } else {
            methods[method].hashes[hash] = true;
        }
    }
}


console.log("Results:");
for (method in methods) {
    console.log(method, ":", methods[method].collision_count, "collisions");
}