llama_cpp_bindings/
llguidance_sampler.rs

1//! Pure Rust llguidance sampler for constrained decoding.
2//!
3//! Implements a custom `llama_sampler` using the `llguidance` and `toktrie` Rust crates
4//! to enforce grammar constraints (JSON schema, regex, Lark, etc.) during token sampling.
5
6use std::ffi::c_void;
7use std::sync::Arc;
8
9use llguidance::Matcher;
10use toktrie::ApproximateTokEnv;
11
12use crate::GrammarError;
13use crate::model::LlamaModel;
14use crate::sampling::LlamaSampler;
15
16/// Internal state for the llguidance sampler.
17struct LlgContext {
18    matcher: Matcher,
19    tok_env: Arc<ApproximateTokEnv>,
20    grammar_kind: String,
21    grammar_data: String,
22}
23
24const unsafe extern "C" fn llg_name(
25    _smpl: *const llama_cpp_bindings_sys::llama_sampler,
26) -> *const std::os::raw::c_char {
27    c"llguidance".as_ptr()
28}
29
30unsafe extern "C" fn llg_accept(
31    smpl: *mut llama_cpp_bindings_sys::llama_sampler,
32    token: llama_cpp_bindings_sys::llama_token,
33) {
34    let ctx = unsafe { &mut *(*smpl).ctx.cast::<LlgContext>() };
35
36    if let Err(consume_error) = ctx.matcher.consume_token(token.cast_unsigned()) {
37        log::warn!(
38            "llguidance sampler failed to consume token: token={token}, error={consume_error}",
39        );
40    }
41}
42
43unsafe extern "C" fn llg_apply(
44    smpl: *mut llama_cpp_bindings_sys::llama_sampler,
45    cur_p: *mut llama_cpp_bindings_sys::llama_token_data_array,
46) {
47    let ctx = unsafe { &mut *(*smpl).ctx.cast::<LlgContext>() };
48    let cur_p = unsafe { &mut *cur_p };
49
50    let mask = match ctx.matcher.compute_mask() {
51        Ok(mask) => mask,
52        Err(compute_error) => {
53            log::warn!(
54                "llguidance sampler failed to compute mask, skipping constraint application: error={compute_error}",
55            );
56
57            return;
58        }
59    };
60
61    let data = unsafe { std::slice::from_raw_parts_mut(cur_p.data, cur_p.size) };
62    for item in data.iter_mut() {
63        if !mask.is_allowed(item.id.cast_unsigned()) {
64            item.logit = f32::NEG_INFINITY;
65        }
66    }
67}
68
69unsafe extern "C" fn llg_reset(smpl: *mut llama_cpp_bindings_sys::llama_sampler) {
70    let ctx = unsafe { &mut *(*smpl).ctx.cast::<LlgContext>() };
71
72    if let Err(reset_error) = ctx.matcher.reset() {
73        log::warn!("llguidance sampler failed to reset: error={reset_error}");
74    }
75}
76
77unsafe extern "C" fn llg_clone(
78    smpl: *const llama_cpp_bindings_sys::llama_sampler,
79) -> *mut llama_cpp_bindings_sys::llama_sampler {
80    let ctx = unsafe { &*(*smpl).ctx.cast::<LlgContext>() };
81    let new_ctx = Box::new(LlgContext {
82        matcher: ctx.matcher.deep_clone(),
83        tok_env: Arc::clone(&ctx.tok_env),
84        grammar_kind: ctx.grammar_kind.clone(),
85        grammar_data: ctx.grammar_data.clone(),
86    });
87    unsafe {
88        llama_cpp_bindings_sys::llama_sampler_init(
89            &raw mut LLG_SAMPLER_I,
90            Box::into_raw(new_ctx).cast::<c_void>(),
91        )
92    }
93}
94
95unsafe extern "C" fn llg_free(smpl: *mut llama_cpp_bindings_sys::llama_sampler) {
96    let ctx_ptr = unsafe { (*smpl).ctx.cast::<LlgContext>() };
97    if !ctx_ptr.is_null() {
98        drop(unsafe { Box::from_raw(ctx_ptr) });
99    }
100}
101
102static mut LLG_SAMPLER_I: llama_cpp_bindings_sys::llama_sampler_i =
103    llama_cpp_bindings_sys::llama_sampler_i {
104        name: Some(llg_name),
105        accept: Some(llg_accept),
106        apply: Some(llg_apply),
107        reset: Some(llg_reset),
108        clone: Some(llg_clone),
109        free: Some(llg_free),
110        backend_init: None,
111        backend_accept: None,
112        backend_apply: None,
113        backend_set_input: None,
114    };
115
116/// Create an llguidance-based constrained decoding sampler.
117///
118/// # Errors
119///
120/// Returns `GrammarError` if the parser factory, grammar, or parser cannot be created.
121pub fn create_llg_sampler(
122    model: &LlamaModel,
123    grammar_kind: &str,
124    grammar_data: &str,
125) -> Result<LlamaSampler, GrammarError> {
126    let tok_env = model.approximate_tok_env();
127    let tok_env_dyn: Arc<dyn toktrie::TokenizerEnv + Sync> = tok_env.clone();
128
129    let factory = llguidance::ParserFactory::new_simple(&tok_env_dyn)
130        .map_err(|factory_error| GrammarError::LlguidanceError(factory_error.to_string()))?;
131
132    let grammar = llguidance::api::TopLevelGrammar::from_tagged_str(grammar_kind, grammar_data)
133        .map_err(|parse_error| GrammarError::LlguidanceError(parse_error.to_string()))?;
134
135    let parser = factory
136        .create_parser(grammar)
137        .map_err(|parser_error| GrammarError::LlguidanceError(parser_error.to_string()))?;
138
139    let matcher = Matcher::new(Ok(parser));
140
141    let ctx = Box::new(LlgContext {
142        matcher,
143        tok_env,
144        grammar_kind: grammar_kind.to_string(),
145        grammar_data: grammar_data.to_string(),
146    });
147
148    let sampler = unsafe {
149        llama_cpp_bindings_sys::llama_sampler_init(
150            &raw mut LLG_SAMPLER_I,
151            Box::into_raw(ctx).cast::<c_void>(),
152        )
153    };
154
155    if sampler.is_null() {
156        Err(GrammarError::LlguidanceSamplerUnavailable)
157    } else {
158        Ok(LlamaSampler { sampler })
159    }
160}
llama_cpp_bindings/llguidance_sampler.rs

llama_cpp_bindings/
llguidance_sampler.rs
