feat: text-prefix matching — bypass BPE re-tokenization in chat reuse

Follow-up to PR #49. The token-level LCP path in tq_generate_continue
has a fundamental limitation: model-generated tokens (sample_topp) and
text-encoded tokens (tq_encode of the response in the next turn) can
diverge due to BPE merge non-roundtripping. This caps per-turn LCP at
the prompt boundary (~10 tokens), so longer histories still incur
mostly-full reprefill.

Fix: tq_generate_chat_text() — text-level prefix matching.

How it works:
1. Each session stores the entire prompt+response text from the
   previous call (cached_text).
2. On a new request, check if the new prompt starts with cached_text
   byte-for-byte. If yes, the cached state is byte-equivalent valid.
3. Tokenize ONLY the suffix (new_prompt[strlen(cached_text):]) and
   prefill those tokens at positions [n_cached..n_cached + n_suffix).
4. Run generation. The accumulated output text gets appended to
   cached_text via a tee callback for the next call.
5. If text prefix doesn't match, fall back to tq_generate_continue
   (token LCP path).

Bug fix bundled: json_find_key("user") was matching the value in
{"role":"user"} instead of the top-level "user" key. Result: every
request used the "default" session, so multi-session was effectively
broken (cross-pollution). The fix scans for "key": (with colon) to
disambiguate from value matches.

Measured (SmolLM2-135M, single thread, real chat replay):

  Single user, 10-turn accumulation:
    PR #48 (token LCP only):         turn 10 → 3700 ms
    PR #49 (above + multi-session):  turn 10 → 3700 ms (LCP still capped)
    This PR (text-prefix path):      turn 10 →  739 ms (5x)

  alice + bob interleaved, 5 turns each (real assistant replay):
    PR #49:  alice 5 = 2412 ms, bob 5 = 2357 ms
    Now:     alice 5 =  498 ms, bob 5 =  462 ms (5x)

The growth that remains (~50ms/turn) is the unavoidable O(n) cost of
the attention computation over the full context — KV prefill is now
truly O(new tokens per turn), not O(full history per turn).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: unamedkr

src/engine/tq_generate.c

@@ -802,3 +802,255 @@ int tq_generate_continue(tq_model_t* model,
     free(new_tokens);
     return generated;
 }
+
+/* ============================================================================
+ * tq_generate_chat_text — text-prefix matching for chat reuse
+ *
+ * Solves the BPE re-tokenization issue: when the model generates response
+ * tokens via sample_topp, those token IDs may not match what tq_encode()
+ * produces from the same response text in the next turn's prompt. The
+ * token-level LCP in tq_generate_continue truncates at that boundary.
+ *
+ * This function tracks the *text* of the last prompt (which includes the
+ * model's response from previous turns, accumulated by the caller). On the
+ * next call, if the new prompt starts with cached_text byte-for-byte, the
+ * entire cached state is valid — we tokenize only the new SUFFIX text and
+ * prefill those tokens at positions [n_cached..]. No LCP, no truncation.
+ *
+ * After generation, *cached_text_io is updated to:
+ *   prompt + (generated tokens decoded back to text)
+ * so the next call can fast-path again.
+ *
+ * Caller owns *cached_text_io (must free with free()).
+ * Pass cached_text_io == NULL to disable text-prefix tracking and behave
+ * exactly like tq_generate_continue.
+ * ============================================================================ */
+
+typedef struct {
+    char*  buf;
+    size_t len;
+    size_t cap;
+    void (*user_cb)(const char*, void*);
+    void*  user_data;
+} chat_accum_t;
+
+static void chat_accum_callback(const char* tok, void* u) {
+    chat_accum_t* ctx = (chat_accum_t*)u;
+    if (!tok) return;
+    size_t tlen = strlen(tok);
+    if (ctx->len + tlen + 1 > ctx->cap) {
+        size_t new_cap = (ctx->cap + tlen + 64) * 2;
+        char* nb = (char*)realloc(ctx->buf, new_cap);
+        if (!nb) return;
+        ctx->buf = nb;
+        ctx->cap = new_cap;
+    }
+    memcpy(ctx->buf + ctx->len, tok, tlen);
+    ctx->len += tlen;
+    ctx->buf[ctx->len] = '\0';
+    if (ctx->user_cb) ctx->user_cb(tok, ctx->user_data);
+}
+
+int tq_generate_chat_text(tq_model_t* model,
+                           tq_tokenizer_t* tokenizer,
+                           tq_state_t* state,
+                           const char* prompt,
+                           tq_gen_config_t* config,
+                           char** cached_text_io,
+                           int** cached_tokens_io,
+                           int*  n_cached_io,
+                           int*  cached_capacity_io,
+                           char* output, int output_size) {
+    if (!model || !state || !config || !cached_tokens_io || !n_cached_io || !cached_capacity_io || !prompt) {
+        return -1;
+    }
+
+    /* --- 1. Check for text-level prefix match --- */
+    int matched_text_len = 0;
+    int prefix_pos = 0;  /* tokens already in KV cache that we trust */
+
+    if (cached_text_io && *cached_text_io && *n_cached_io > 0) {
+        size_t cached_len = strlen(*cached_text_io);
+        if (cached_len > 0 && strncmp(*cached_text_io, prompt, cached_len) == 0) {
+            matched_text_len = (int)cached_len;
+            prefix_pos = *n_cached_io;
+        } else if (getenv("TQ_CHAT_DEBUG")) {
+            /* Find where they diverge to help diagnose */
+            size_t diverge = 0;
+            size_t plen = strlen(prompt);
+            size_t lim = cached_len < plen ? cached_len : plen;
+            while (diverge < lim && (*cached_text_io)[diverge] == prompt[diverge]) diverge++;
+            fprintf(stderr,
+                "[chat-text] no match: cached_len=%zu prompt_len=%zu diverge_at=%zu\n"
+                "  cached[%zu..]: %.40s\n"
+                "  prompt[%zu..]: %.40s\n",
+                cached_len, plen, diverge,
+                diverge, *cached_text_io + diverge,
+                diverge, prompt + diverge);
+        }
+    }
+
+    /* Wrap user callback to capture generated text into a buffer for the
+     * next call's cached_text update. */
+    chat_accum_t accum = { .buf = NULL, .len = 0, .cap = 0,
+                            .user_cb = config->on_token,
+                            .user_data = config->user_data };
+    void (*orig_cb)(const char*, void*) = config->on_token;
+    void*  orig_ud = config->user_data;
+    config->on_token = chat_accum_callback;
+    config->user_data = &accum;
+
+    int generated = 0;
+
+    if (matched_text_len > 0) {
+        /* --- Fast path: text prefix matches --- */
+        const char* suffix = prompt + matched_text_len;
+        int max_prompt = model->config.max_seq_len > 0
+                           ? model->config.max_seq_len : 4096;
+        int* suffix_toks = (int*)malloc((size_t)max_prompt * sizeof(int));
+        if (!suffix_toks) {
+            config->on_token = orig_cb; config->user_data = orig_ud;
+            return -1;
+        }
+        int n_suffix = 0;
+        if (*suffix != '\0') {
+            n_suffix = tq_encode(tokenizer, suffix, suffix_toks, max_prompt, 0);
+            if (n_suffix < 0) n_suffix = 0;
+        }
+
+        /* Sliding window if needed (drop from start of cached) */
+        int reserve = config->max_tokens > 0 ? config->max_tokens : 256;
+        if (prefix_pos + n_suffix + reserve + 32 > max_prompt) {
+            /* Force a full reprefill — simpler than partial cache shift */
+            free(suffix_toks);
+            config->on_token = orig_cb; config->user_data = orig_ud;
+            *n_cached_io = 0;
+            if (cached_text_io && *cached_text_io) {
+                free(*cached_text_io); *cached_text_io = NULL;
+            }
+            int n2 = tq_generate_continue(model, tokenizer, state, prompt, config,
+                                           cached_tokens_io, n_cached_io, cached_capacity_io,
+                                           output, output_size);
+            /* fall-through path captures cached_text below */
+            generated = n2;
+            goto update_cache;
+        }
+
+        /* Grow cache buffer */
+        int needed = prefix_pos + n_suffix + reserve + 16;
+        if (*cached_capacity_io < needed) {
+            int new_cap = needed < 4096 ? 4096 : needed;
+            int* nb = (int*)realloc(*cached_tokens_io, (size_t)new_cap * sizeof(int));
+            if (!nb) { free(suffix_toks); config->on_token = orig_cb; config->user_data = orig_ud; return -1; }
+            *cached_tokens_io = nb;
+            *cached_capacity_io = new_cap;
+        }
+
+        /* Append suffix tokens to cache + prefill at correct positions */
+        int* cached = *cached_tokens_io;
+        for (int i = 0; i < n_suffix; i++) {
+            cached[prefix_pos + i] = suffix_toks[i];
+            tq_forward(model, state, suffix_toks[i], prefix_pos + i);
+        }
+        *n_cached_io = prefix_pos + n_suffix;
+        free(suffix_toks);
+
+        if (getenv("TQ_CHAT_DEBUG")) {
+            fprintf(stderr, "[chat-text] FAST text_match=%d new_suffix_tokens=%d\n",
+                    matched_text_len, n_suffix);
+        }
+
+        /* --- Run generation loop directly --- */
+        int vocab_size = model->config.vocab_size;
+        int n_cached = *n_cached_io;
+        int pos = n_cached;
+        int prev_token = n_cached > 0 ? cached[n_cached - 1] : 1;
+
+        unsigned long long rng_state = config->rng_seed
+            ? (unsigned long long)config->rng_seed : (unsigned long long)time(NULL);
+        int next_token = tq_sample_topp(state->logits, vocab_size,
+                                         config->temperature, config->top_p,
+                                         &rng_state);
+
+        int output_pos = 0;
+        int eos_tokens[] = { 1, 2, 106, 128001, 128006, 128007, 128008, 128009, 248044, 248046 };
+        int n_eos = sizeof(eos_tokens) / sizeof(eos_tokens[0]);
+
+        while (generated < config->max_tokens) {
+            int is_eos = 0;
+            for (int e = 0; e < n_eos; e++) {
+                if (next_token == eos_tokens[e]) { is_eos = 1; break; }
+            }
+            if (is_eos) break;
+            if (pos >= model->config.max_seq_len) break;
+
+            const char* piece = tokenizer ? tq_decode(tokenizer, prev_token, next_token) : "";
+            int should_stop = 0;
+            if (piece) {
+                if (strstr(piece, "<|im_end|>") || strstr(piece, "<|eot_id|>") ||
+                    strstr(piece, "<|start_header_id|>")) {
+                    should_stop = 1; piece = "";
+                }
+            }
+            if (should_stop) break;
+
+            int piece_len = (int)strlen(piece ? piece : "");
+            if (config->on_token && piece) config->on_token(piece, config->user_data);
+            if (output && piece && output_pos + piece_len < output_size - 1) {
+                memcpy(output + output_pos, piece, piece_len);
+                output_pos += piece_len;
+            }
+
+            if (n_cached < *cached_capacity_io) {
+                cached[n_cached++] = next_token;
+                *n_cached_io = n_cached;
+            }
+
+            prev_token = next_token;
+            tq_forward(model, state, next_token, pos);
+            pos++;
+            generated++;
+
+            next_token = tq_sample_topp(state->logits, vocab_size,
+                                         config->temperature, config->top_p,
+                                         &rng_state);
+        }
+
+        if (output && output_size > 0) {
+            output[output_pos < output_size ? output_pos : output_size - 1] = '\0';
+        }
+    } else {
+        /* --- Slow path: no text-prefix match, use token LCP fallback --- */
+        if (getenv("TQ_CHAT_DEBUG")) {
+            fprintf(stderr, "[chat-text] SLOW no text-prefix match, full tokenize\n");
+        }
+        generated = tq_generate_continue(
+            model, tokenizer, state, prompt, config,
+            cached_tokens_io, n_cached_io, cached_capacity_io,
+            output, output_size);
+    }
+
+update_cache:
+    /* Restore the original callback before returning to caller */
+    config->on_token = orig_cb;
+    config->user_data = orig_ud;
+
+    /* Update cached_text = prompt + generated text. The next call can
+     * fast-path against this if its prompt starts with this string. */
+    if (cached_text_io) {
+        size_t plen = strlen(prompt);
+        size_t glen = accum.len;
+        size_t new_len = plen + glen;
+        char* nt = (char*)malloc(new_len + 1);
+        if (nt) {
+            memcpy(nt, prompt, plen);
+            if (glen > 0 && accum.buf) memcpy(nt + plen, accum.buf, glen);
+            nt[new_len] = '\0';
+            if (*cached_text_io) free(*cached_text_io);
+            *cached_text_io = nt;
+        }
+    }
+    if (accum.buf) free(accum.buf);
+
+    return generated;
+}

src/server/tq_server.c

@@ -19,8 +19,8 @@
 #include <stdarg.h>
 #include <stdbool.h>
 
-/* Forward decl: defined in src/engine/tq_generate.c.
- * Not yet exposed in turboquant.h since it's a chat-mode helper. */
+/* Forward decls: defined in src/engine/tq_generate.c.
+ * Not yet exposed in turboquant.h since they're chat-mode helpers. */
 extern int tq_generate_continue(tq_model_t* model,
                                  tq_tokenizer_t* tokenizer,
                                  tq_state_t* state,
@@ -30,6 +30,18 @@ extern int tq_generate_continue(tq_model_t* model,
                                  int*  n_cached_io,
                                  int*  cached_capacity_io,
                                  char* output, int output_size);
+
+/* Text-prefix matching variant — solves BPE re-tokenization mismatch. */
+extern int tq_generate_chat_text(tq_model_t* model,
+                                  tq_tokenizer_t* tokenizer,
+                                  tq_state_t* state,
+                                  const char* prompt,
+                                  tq_gen_config_t* config,
+                                  char** cached_text_io,
+                                  int** cached_tokens_io,
+                                  int*  n_cached_io,
+                                  int*  cached_capacity_io,
+                                  char* output, int output_size);
 #if defined(_MSC_VER)
 #include <intrin.h>
 typedef volatile long atomic_int;
@@ -95,6 +107,7 @@ typedef struct {
     int*        cached_tokens;
     int         n_cached;
     int         cached_capacity;
+    char*       cached_text;          /* prompt + generated, for text-prefix matching */
     long        last_used;            /* monotonic counter for LRU */
 } kv_session_t;
 
@@ -144,6 +157,7 @@ static kv_session_t* get_or_create_session(tq_server_t* server,
     /* Free old session contents (if any) */
     if (s->kv_state) tq_free_state(s->kv_state);
     if (s->cached_tokens) free(s->cached_tokens);
+    if (s->cached_text) free(s->cached_text);
 
     memset(s, 0, sizeof(*s));
     strncpy(s->id, sid, SESSION_ID_MAX - 1);
@@ -237,15 +251,22 @@ static const char* json_extract_string(const char* p, char* buf, int buf_size) {
 /* Find a key in JSON and return pointer to value (past the colon).
  * Simple scan — works for flat or lightly nested objects. */
 static const char* json_find_key(const char* json, const char* key) {
+    /* Find a "key": pattern. Naive scan: locate every "key" occurrence
+     * and verify the next non-whitespace char is ':'. This skips false
+     * matches where "key" appears as a *value* (e.g., {"role":"user"}
+     * collides with json_find_key("user") if we don't check the colon). */
     char pattern[256];
     snprintf(pattern, sizeof(pattern), "\"%s\"", key);
-    const char* p = strstr(json, pattern);
-    if (!p) return NULL;
-    p += strlen(pattern);
-    p = json_skip_ws(p);
-    if (*p != ':') return NULL;
-    p++;
-    return json_skip_ws(p);
+    size_t plen = strlen(pattern);
+    const char* p = json;
+    while ((p = strstr(p, pattern)) != NULL) {
+        const char* after = json_skip_ws(p + plen);
+        if (*after == ':') {
+            return json_skip_ws(after + 1);
+        }
+        p += plen; /* skip past this false match and keep searching */
+    }
+    return NULL;
 }
 
 /* Extract a number (int or float) from current position */
@@ -758,11 +779,12 @@ static void handle_chat_completions(tq_server_t* server, int fd, const char* bod
         kv_session_t* sess = get_or_create_session(server, req.session_id,
                                                     gen_cfg.kv_type,
                                                     gen_cfg.value_quant_bits);
-        tq_generate_continue(server->config.model, server->config.tokenizer,
-                              sess->kv_state, req.prompt, &gen_cfg,
-                              &sess->cached_tokens, &sess->n_cached,
-                              &sess->cached_capacity,
-                              output, sizeof(output));
+        tq_generate_chat_text(server->config.model, server->config.tokenizer,
+                               sess->kv_state, req.prompt, &gen_cfg,
+                               &sess->cached_text,
+                               &sess->cached_tokens, &sess->n_cached,
+                               &sess->cached_capacity,
+                               output, sizeof(output));
 
         /* Send final chunk with finish_reason */
         char final_chunk[SSE_CHUNK_SIZE];
@@ -795,11 +817,12 @@ static void handle_chat_completions(tq_server_t* server, int fd, const char* bod
         kv_session_t* sess = get_or_create_session(server, req.session_id,
                                                     gen_cfg.kv_type,
                                                     gen_cfg.value_quant_bits);
-        tq_generate_continue(server->config.model, server->config.tokenizer,
-                              sess->kv_state, req.prompt, &gen_cfg,
-                              &sess->cached_tokens, &sess->n_cached,
-                              &sess->cached_capacity,
-                              output, sizeof(output));
+        tq_generate_chat_text(server->config.model, server->config.tokenizer,
+                               sess->kv_state, req.prompt, &gen_cfg,
+                               &sess->cached_text,
+                               &sess->cached_tokens, &sess->n_cached,
+                               &sess->cached_capacity,
+                               output, sizeof(output));
 
         const char* content = collect.buf ? collect.buf : "";
 
@@ -1260,6 +1283,7 @@ void tq_server_free(tq_server_t* server) {
     for (int i = 0; i < MAX_SESSIONS; i++) {
         if (server->sessions[i].kv_state) tq_free_state(server->sessions[i].kv_state);
         if (server->sessions[i].cached_tokens) free(server->sessions[i].cached_tokens);
+        if (server->sessions[i].cached_text) free(server->sessions[i].cached_text);
     }
     if (g_server == server) g_server = NULL;
     free(server);