🧪 Add missing tests for Engine inference functions

echo-core-zig/src/inference/engine.zig

@@ -1016,3 +1016,57 @@ test "Engine.greedyNextToken correctly identifies token with max logit" {
     eng.logits[0] = 10.0;
     try std.testing.expectEqual(@as(u32, 0), eng.greedyNextToken());
 }
+
+test "Engine.greedyNextToken handles all negative logits" {
+    const cfg = makeTinyConfig(0, 0);
+    var eng = try Engine.init(cfg, null, std.testing.allocator);
+    defer eng.deinit(std.testing.allocator);
+
+    @memset(eng.logits, -1000.0);
+    eng.logits[0] = -10.0;
+    eng.logits[1] = -5.0;
+    eng.logits[2] = -20.0;
+    eng.logits[3] = -100.0;
+
+    // -5.0 is the largest (closest to 0)
+    try std.testing.expectEqual(@as(u32, 1), eng.greedyNextToken());
+}
+
+test "Engine.decodeStep advances seq_pos and returns logits" {
+    const cfg = makeTinyConfig(1, 8);
+    var eng = try Engine.init(cfg, null, std.testing.allocator);
+    defer eng.deinit(std.testing.allocator);
+
+    // Initialize weight_pool to zeros
+    @memset(eng.weight_pool, 0);
+
+    try std.testing.expectEqual(@as(u32, 0), eng.seq_pos);
+
+    const logits1 = try eng.decodeStep(0);
+    try std.testing.expectEqual(@as(usize, cfg.vocab_size), logits1.len);
+    try std.testing.expectEqual(@as(u32, 1), eng.seq_pos);
+
+    const logits2 = try eng.decodeStep(1);
+    try std.testing.expectEqual(@as(usize, cfg.vocab_size), logits2.len);
+    try std.testing.expectEqual(@as(u32, 2), eng.seq_pos);
+}
+
+test "Engine.prefill processes multiple tokens and advances seq_pos correctly" {
+    const cfg = makeTinyConfig(1, 8);
+    var eng = try Engine.init(cfg, null, std.testing.allocator);
+    defer eng.deinit(std.testing.allocator);
+
+    @memset(eng.weight_pool, 0);
+
+    try std.testing.expectEqual(@as(u32, 0), eng.seq_pos);
+
+    const prompt = [_]u32{ 0, 1, 2 };
+    const logits = try eng.prefill(&prompt);
+
+    try std.testing.expectEqual(@as(usize, cfg.vocab_size), logits.len);
+    try std.testing.expectEqual(@as(u32, 3), eng.seq_pos);
+
+    // Check that KV cache actually registered the tokens
+    try std.testing.expect(eng.kv_cache != null);
+    try std.testing.expectEqual(@as(u32, 3), eng.kv_cache.?.seqLen());
+}