diff --git a/run.c b/run.c
index 912c3cb..477011c 100644
--- a/run.c
+++ b/run.c
@@ -337,6 +337,9 @@ void transformer(int token, int pos, Config* p, RunState* s, TransformerWeights*
     matmul(s->logits, x, w->wcls, p->dim, p->vocab_size);
 }
 
+// ----------------------------------------------------------------------------
+// functions to sample the next token from the transformer's predicted distribution
+
 int sample(float* probabilities, int n) {
     // sample index from probabilities, they must sum to 1
     float r = (float)rand() / (float)RAND_MAX;
@@ -362,14 +365,76 @@ int argmax(float* v, int n) {
     }
     return max_i;
 }
+// ----------------------------------------------------------------------------
+// byte pair encoding (BPE) tokenizer, encodes strings into tokens so we can prompt
+
+int str_lookup(char *str, char **vocab, int vocab_size) {
+    // find the first perfect match for str in vocab, return its index or -1 if not found
+    for (int i = 0; i < vocab_size; i++) {
+        if (strcmp(str, vocab[i]) == 0) {
+            return i;
+        }
+    }
+    return -1;
+}
+
+void bpe_encode(char *text, char **vocab, float *vocab_scores, int vocab_size, unsigned int max_token_length, int *tokens, int *n_tokens) {
+    
+    // a temporary buffer to merge two consecutive tokens
+    char* str_buffer = malloc((max_token_length*2+1) * sizeof(char)); // *2 for concat, +1 for null terminator
+
+    // first encode every individual byte in the input string
+    *n_tokens = 0; // the number of tokens
+    for (char *c = text; *c != '\0'; c++) {
+        sprintf(str_buffer, "%c", *c);
+        int id = str_lookup(str_buffer, vocab, vocab_size);
+        if (id == -1) { printf("not good\n"); exit(1);}
+        tokens[*n_tokens] = id;
+        (*n_tokens)++;
+    }
+
+    // merge the best consecutive pair each iteration, according the scores in vocab_scores
+    while (1) {
+        float best_score = -1e10;
+        int best_id = -1;
+        int best_idx = -1;
+
+        for (int i=0; i < (*n_tokens-1); i++) {
+            // check if we can merge the pair (tokens[i], tokens[i+1])
+            sprintf(str_buffer, "%s%s", vocab[tokens[i]], vocab[tokens[i+1]]);
+            int id = str_lookup(str_buffer, vocab, vocab_size);
+            if (id != -1 && vocab_scores[id] > best_score) {
+                // this merge pair exists in vocab! record its score and position
+                best_score = vocab_scores[id];
+                best_id = id;
+                best_idx = i;
+            }
+        }
+
+        if (best_idx == -1) {
+            break; // we couldn't find any more pairs to merge, so we're done
+        }
+
+        // merge the consecutive pair (best_idx, best_idx+1) into new token best_id
+        tokens[best_idx] = best_id;
+        // delete token at position best_idx+1, shift the entire sequence back 1
+        for (int i = best_idx+1; i < (*n_tokens-1); i++) {
+            tokens[i] = tokens[i+1];
+        }
+        (*n_tokens)--; // token length decreased
+    }
+
+    free(str_buffer);
+}
 
 // ----------------------------------------------------------------------------
-
+// utilities
 long time_in_ms() {
     struct timespec time;
     clock_gettime(CLOCK_REALTIME, &time);
     return time.tv_sec * 1000 + time.tv_nsec / 1000000;
 }
+// ----------------------------------------------------------------------------
 
 int main(int argc, char *argv[]) {
 
@@ -377,9 +442,11 @@ int main(int argc, char *argv[]) {
     char *checkpoint = NULL;  // e.g. out/model.bin
     float temperature = 0.9f; // e.g. 1.0, or 0.0
     int steps = 256;          // max number of steps to run for, 0: use seq_len
+    char *prompt = NULL;      // prompt string
+
     // 'checkpoint' is necessary arg
     if (argc < 2) {
-        printf("Usage: %s <checkpoint_file> [temperature] [steps]\n", argv[0]);
+        printf("Usage: %s <checkpoint_file> [temperature] [steps] [prompt]\n", argv[0]);
         return 1;
     }
     if (argc >= 2) {
@@ -392,6 +459,9 @@ int main(int argc, char *argv[]) {
     if (argc >= 4) {
         steps = atoi(argv[3]);
     }
+    if (argc >= 5) {
+        prompt = argv[4];
+    }
 
     // seed rng with time. if you want deterministic behavior use temperature 0.0
     srand((unsigned int)time(NULL)); 
@@ -406,7 +476,7 @@ int main(int argc, char *argv[]) {
         FILE *file = fopen(checkpoint, "rb");
         if (!file) { printf("Couldn't open file %s\n", checkpoint); return 1; }
         // read in the config header
-        if(fread(&config, sizeof(Config), 1, file) != 1) { return 1; }
+        if (fread(&config, sizeof(Config), 1, file) != 1) { return 1; }
         // negative vocab size is hacky way of signaling unshared weights. bit yikes.
         int shared_weights = config.vocab_size > 0 ? 1 : 0;
         config.vocab_size = abs(config.vocab_size);
@@ -427,14 +497,18 @@ int main(int argc, char *argv[]) {
 
     // read in the tokenizer.bin file
     char** vocab = (char**)malloc(config.vocab_size * sizeof(char*));
+    float* vocab_scores = (float*)malloc(config.vocab_size * sizeof(float));
+    unsigned int max_token_length;
     {
         FILE *file = fopen("tokenizer.bin", "rb");
-        if (!file) { printf("Couldn't load tokenizer.bin\n"); return 1; }
+        if (!file) { printf("couldn't load tokenizer.bin\n"); return 1; }
+        if (fread(&max_token_length, sizeof(int), 1, file) != 1) { printf("failed read\n"); return 1; }
         int len;
         for (int i = 0; i < config.vocab_size; i++) {
-            if(fread(&len, sizeof(int), 1, file) != 1) { return 1; }
+            if (fread(vocab_scores + i, sizeof(float), 1, file) != 1) { printf("failed read\n"); return 1;}
+            if (fread(&len, sizeof(int), 1, file) != 1) { printf("failed read\n"); return 1; }
             vocab[i] = (char *)malloc(len + 1);
-            if(fread(vocab[i], len, 1, file) != 1) { return 1; }
+            if (fread(vocab[i], len, 1, file) != 1) { printf("failed read\n"); return 1; }
             vocab[i][len] = '\0'; // add the string terminating token
         }
         fclose(file);
@@ -443,30 +517,44 @@ int main(int argc, char *argv[]) {
     // create and init the application RunState
     RunState state;
     malloc_run_state(&state, &config);
-    
-    // the current position we are in
-    long start = 0; // used to time our code, only initialized after first iteration
-    int next;
-    int token = 1; // 1 = BOS token in Llama-2 sentencepiece
-    int pos = 0;
-    printf("<s>\n"); // explicit print the initial BOS token (=1), stylistically symmetric
+
+    // process the prompt, if any
+    int *prompt_tokens = NULL;
+    int num_prompt_tokens = 0;
+    if (prompt != NULL) {
+        prompt_tokens = (int*)malloc(config.seq_len * sizeof(int));
+        bpe_encode(prompt, vocab, vocab_scores, config.vocab_size, max_token_length, prompt_tokens, &num_prompt_tokens);
+    }
+
+    // start the main loop
+    long start = 0;  // used to time our code, only initialized after first iteration
+    int next;        // will store the next token in the sequence
+    int token = 1;   // init with token 1 (=BOS), as done in Llama-2 sentencepiece tokenizer
+    int pos = 0;     // position in the sequence
+    printf("<s>\n"); // explicit print the initial BOS token for stylistic symmetry reasons
     while (pos < steps) {
 
         // forward the transformer to get logits for the next token
         transformer(token, pos, &config, &state, &weights);
 
-        // sample the next token
-        if(temperature == 0.0f) {
-            // greedy argmax sampling
-            next = argmax(state.logits, config.vocab_size);
+        if(pos < num_prompt_tokens) {
+            // if we are still processing the input prompt, force the next prompt token
+            next = prompt_tokens[pos];
         } else {
-            // apply the temperature to the logits
-            for (int q=0; q<config.vocab_size; q++) { state.logits[q] /= temperature; }
-            // apply softmax to the logits to get the probabilities for next token
-            softmax(state.logits, config.vocab_size);
-            // we now want to sample from this distribution to get the next token
-            next = sample(state.logits, config.vocab_size);
+            // sample the next token
+            if (temperature == 0.0f) {
+                // greedy argmax sampling: take the token with the highest probability
+                next = argmax(state.logits, config.vocab_size);
+            } else {
+                // apply the temperature to the logits
+                for (int q=0; q<config.vocab_size; q++) { state.logits[q] /= temperature; }
+                // apply softmax to the logits to get the probabilities for next token
+                softmax(state.logits, config.vocab_size);
+                // we sample from this distribution to get the next token
+                next = sample(state.logits, config.vocab_size);
+            }
         }
+
         // following BOS token (1), sentencepiece decoder strips any leading whitespace (see PR #89)
         char *token_str = (token == 1 && vocab[next][0] == ' ') ? vocab[next]+1 : vocab[next];
         printf("%s", token_str);
@@ -487,6 +575,8 @@ int main(int argc, char *argv[]) {
     free_run_state(&state);
     for (int i = 0; i < config.vocab_size; i++) { free(vocab[i]); }
     free(vocab);
+    free(vocab_scores);
+    if (prompt_tokens != NULL) free(prompt_tokens);
     if (data != MAP_FAILED) munmap(data, file_size);
     if (fd != -1) close(fd);
     return 0;
diff --git a/tokenizer.bin b/tokenizer.bin
index 18a803f..e0a8a7b 100644
Binary files a/tokenizer.bin and b/tokenizer.bin differ
diff --git a/tokenizer.py b/tokenizer.py
index a147239..35eee20 100644
--- a/tokenizer.py
+++ b/tokenizer.py
@@ -3,6 +3,7 @@
 # This software may be used and distributed according to the terms of the Llama 2 Community License Agreement.
 
 import os
+import struct
 from logging import getLogger
 from typing import List
 
@@ -39,26 +40,35 @@ class Tokenizer:
         return self.sp_model.decode(t)
 
     def export(self):
-        tokens = []
+
+        # get all the tokens (postprocessed) and their scores as floats
+        tokens, scores = [], []
         for i in range(self.n_words):
 
             # decode the token and light postprocessing
             t = self.sp_model.id_to_piece(i)
+            s = self.sp_model.get_score(i)
             if i == self.bos_id:
                 t = '\n<s>\n'
             elif i == self.eos_id:
                 t = '\n</s>\n'
             elif len(t) == 6 and t.startswith('<0x') and t.endswith('>'):
                 t = chr(int(t[3:5], 16)) # e.g. make '<0x01>' into '\x01'
-            t = t.replace('▁', ' ') # sentencepiece uses this as the whitespace
+            t = t.replace('▁', ' ') # sentencepiece uses this character as whitespace
+            b = t.encode('utf-8') # bytes of this token, utf-8 encoded
 
-            tokens.append(t)
+            tokens.append(b)
+            scores.append(s)
         
+        # record the max token length
+        max_token_length = max(len(t) for t in tokens)
+
+        # write to a binary file
         with open(TOKENIZER_BIN, 'wb') as f:
-            for token in tokens:
-                bytes = token.encode('utf-8')
-                f.write((len(bytes)).to_bytes(4, 'little'))  # write length of bytes
-                f.write(bytes)  # write token bytes
+            f.write(struct.pack("I", max_token_length))
+            for bytes, score in zip(tokens, scores):
+                f.write(struct.pack("fI", score, len(bytes)))
+                f.write(bytes)
 
 if __name__ == "__main__":
     t = Tokenizer()