diff --git a/src/alias.js b/src/alias.js
new file mode 100644
index 0000000..63f44ea
--- /dev/null
+++ b/src/alias.js
@@ -0,0 +1,30 @@
+// src/alias.js
+// Pure functions for deriving snake_case aliases from sprite filenames.
+// No I/O.
+
+const path = require('node:path');
+const SUPPORTED_EXT = ['.png', '.webp', '.jpg', '.jpeg'];
+
+function stripPrefix(name, prefix) {
+    if (!prefix) return name;
+    if (name.startsWith(prefix)) return name.slice(prefix.length);
+    return name;
+}
+
+// CamelCase -> snake_case. Boundary: lowercase/digit -> uppercase.
+function camelToSnakeCase(s) {
+    return s
+        .replace(/([a-z0-9])([A-Z])/g, '$1_$2')
+        .toLowerCase();
+}
+
+function aliasFromFilename(filename, opts = {}) {
+    const ext = path.extname(filename).toLowerCase();
+    let base = SUPPORTED_EXT.includes(ext)
+        ? filename.slice(0, -ext.length)
+        : filename;
+    if (opts.stripPrefix) base = stripPrefix(base, opts.stripPrefix);
+    return camelToSnakeCase(base);
+}
+
+module.exports = { stripPrefix, camelToSnakeCase, aliasFromFilename };
diff --git a/tests/alias.test.js b/tests/alias.test.js
new file mode 100644
index 0000000..7d7bd2c
--- /dev/null
+++ b/tests/alias.test.js
@@ -0,0 +1,50 @@
+const { test } = require('node:test');
+const assert = require('node:assert');
+const { stripPrefix, camelToSnakeCase, aliasFromFilename } = require('../src/alias');
+
+test('stripPrefix: strips matching prefix', () => {
+    assert.strictEqual(stripPrefix('TC_Basics Asset Pack_Bed1', 'TC_Basics Asset Pack_'), 'Bed1');
+});
+
+test('stripPrefix: returns unchanged when no match', () => {
+    assert.strictEqual(stripPrefix('Bed1', 'TC_Basics Asset Pack_'), 'Bed1');
+});
+
+test('stripPrefix: empty prefix returns unchanged', () => {
+    assert.strictEqual(stripPrefix('Bed1', ''), 'Bed1');
+});
+
+test('camelToSnakeCase: BenchTable1 -> bench_table1', () => {
+    assert.strictEqual(camelToSnakeCase('BenchTable1'), 'bench_table1');
+});
+
+test('camelToSnakeCase: ArmourStand -> armour_stand', () => {
+    assert.strictEqual(camelToSnakeCase('ArmourStand'), 'armour_stand');
+});
+
+test('camelToSnakeCase: Bed1 -> bed1', () => {
+    assert.strictEqual(camelToSnakeCase('Bed1'), 'bed1');
+});
+
+test('camelToSnakeCase: BarrelLarge -> barrel_large', () => {
+    assert.strictEqual(camelToSnakeCase('BarrelLarge'), 'barrel_large');
+});
+
+test('camelToSnakeCase: already snake_case unchanged', () => {
+    assert.strictEqual(camelToSnakeCase('bench_table1'), 'bench_table1');
+});
+
+test('aliasFromFilename: full pipeline', () => {
+    const alias = aliasFromFilename('TC_Basics Asset Pack_BenchTable1.png', {
+        stripPrefix: 'TC_Basics Asset Pack_',
+    });
+    assert.strictEqual(alias, 'bench_table1');
+});
+
+test('aliasFromFilename: no opts -> bare-name snake_case', () => {
+    assert.strictEqual(aliasFromFilename('Bed1.png', {}), 'bed1');
+});
+
+test('aliasFromFilename: handles .webp extension', () => {
+    assert.strictEqual(aliasFromFilename('Bed1.webp', {}), 'bed1');
+});