lakehouse/crates/aibridge/src/continuation.rs

//! Phase 21 — OUTPUT-overflow handler. Ports `generateContinuable`
//! from `tests/multi-agent/agent.ts`.
//!
//! Two failure modes to repair:
//!
//! * EMPTY response — thinking model ate the entire budget on hidden
//!   reasoning before emitting a token. Fix: retry the original prompt
//!   with 2× the budget, geometric up to `BUDGET_CAP`.
//!
//! * TRUNCATED non-empty — model got most of the way but hit
//!   max_tokens before closing the structure. Fix: continue with the
//!   partial response in the prompt as scratchpad, so the model knows
//!   where to pick up without restarting.
//!
//! `TextGenerator` abstracts the sidecar so tests can inject canned
//! responses without a live Ollama.

use std::future::Future;

use crate::client::{AiClient, GenerateRequest, GenerateResponse};

/// Shape classifier for `is_structurally_complete`. JSON responses
/// must parse; text responses just need to be non-empty.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ResponseShape {
    Json,
    Text,
}

/// Trait that `generate_continuable` + `generate_tree_split` call. The
/// real implementation forwards to `AiClient::generate`; tests supply a
/// mock with a scripted sequence of responses.
pub trait TextGenerator: Send + Sync {
    fn generate_text(
        &self,
        req: GenerateRequest,
    ) -> impl Future<Output = Result<GenerateResponse, String>> + Send;
}

impl TextGenerator for AiClient {
    fn generate_text(
        &self,
        req: GenerateRequest,
    ) -> impl Future<Output = Result<GenerateResponse, String>> + Send {
        self.generate(req)
    }
}

/// Strip a surrounding ```json``` fence if present. Leaves the inner
/// content alone otherwise. Returns a slice of `s`.
fn strip_json_fence(s: &str) -> &str {
    let t = s.trim();
    if let Some(rest) = t.strip_prefix("```json") {
        rest.trim_start_matches('\n').strip_suffix("```").unwrap_or(rest).trim()
    } else if let Some(rest) = t.strip_prefix("```") {
        rest.trim_start_matches('\n').strip_suffix("```").unwrap_or(rest).trim()
    } else {
        t
    }
}

/// Port of the TS brace-balance + JSON.parse check. Returns true when
/// the outermost `{...}` block is balanced and parses. Text shape is
/// satisfied by any non-empty, non-whitespace payload.
pub fn is_structurally_complete(text: &str, shape: ResponseShape) -> bool {
    if text.trim().is_empty() { return false; }
    if shape == ResponseShape::Text { return true; }

    let s = strip_json_fence(text);
    let Some(start) = s.find('{') else { return false };
    let Some(end) = s.rfind('}') else { return false };
    if end <= start { return false; }
    let slice = &s[start..=end];

    // Balance check — cheaper than parse, catches truncated nests.
    // String state tracked because `{` inside a string doesn't count.
    let mut depth: i32 = 0;
    let mut in_str = false;
    let mut esc = false;
    for c in slice.chars() {
        if esc { esc = false; continue; }
        if c == '\\' { esc = true; continue; }
        if c == '"' { in_str = !in_str; continue; }
        if in_str { continue; }
        if c == '{' { depth += 1; }
        else if c == '}' {
            depth -= 1;
            if depth < 0 { return false; }
        }
    }
    if depth != 0 { return false; }
    // Parse check is the tie-breaker — balanced but invalid JSON (e.g.
    // trailing comma before `}`) shouldn't count as complete.
    serde_json::from_str::<serde_json::Value>(slice).is_ok()
}

/// Knobs for `generate_continuable`. All optional with sensible
/// defaults that match the TS version.
#[derive(Debug, Clone)]
pub struct ContinuableOpts {
    pub model: String,
    pub max_tokens: Option<u32>,
    pub temperature: Option<f64>,
    pub system: Option<String>,
    pub shape: ResponseShape,
    pub max_continuations: usize,
    pub think: Option<bool>,
    /// Geometric-backoff ceiling for the empty-response retry path.
    /// Matches TS's `budgetCap = 8000`.
    pub budget_cap: u32,
    /// Maximum empty-response retries before giving up. Matches TS's
    /// hardcoded `retry < 3`.
    pub max_empty_retries: usize,
}

impl ContinuableOpts {
    pub fn new(model: impl Into<String>) -> Self {
        Self {
            model: model.into(),
            max_tokens: None,
            temperature: None,
            system: None,
            shape: ResponseShape::Json,
            max_continuations: 3,
            think: None,
            budget_cap: 8_000,
            max_empty_retries: 3,
        }
    }
}

/// Outcome of a `generate_continuable` call. Carries the combined
/// text plus diagnostic counters so observability downstream can
/// report "how many continuations did that query cost".
#[derive(Debug, Clone)]
pub struct ContinuableOutcome {
    pub text: String,
    pub empty_retries: usize,
    pub continuations: usize,
    pub final_complete: bool,
}

fn make_request(opts: &ContinuableOpts, prompt: String, current_max: u32) -> GenerateRequest {
    GenerateRequest {
        prompt,
        model: Some(opts.model.clone()),
        system: opts.system.clone(),
        temperature: opts.temperature,
        max_tokens: Some(current_max),
        think: opts.think,
    }
}

fn continuation_prompt(original: &str, partial: &str) -> String {
    format!(
        "{original}\n\n\
         PARTIAL RESPONSE SO FAR (continue from here — do NOT restart, \
         do NOT repeat what's already there, emit ONLY the remaining \
         tokens to close the structure):\n{partial}"
    )
}

/// Phase 21 — output-overflow safe generate. See module docs for the
/// two failure modes repaired. On final-still-incomplete, returns the
/// combined text with `final_complete: false` so the caller's parser
/// can throw with the raw text for forensics rather than silently
/// truncating.
pub async fn generate_continuable<G: TextGenerator>(
    generator: &G,
    prompt: &str,
    opts: &ContinuableOpts,
) -> Result<ContinuableOutcome, String> {
    let initial_max = opts.max_tokens.unwrap_or(800);
    let mut current_max = initial_max;
    let mut combined = String::new();
    let mut empty_retries = 0usize;
    let mut continuations = 0usize;

    // Phase 21(a) — empty-response backoff loop.
    for retry in 0..opts.max_empty_retries {
        let req = make_request(opts, prompt.to_string(), current_max);
        let resp = generator.generate_text(req).await?;
        if !resp.text.trim().is_empty() {
            combined = resp.text;
            break;
        }
        empty_retries = retry + 1;
        current_max = (current_max.saturating_mul(2)).min(opts.budget_cap);
    }

    // Phase 21(b) — structural-completion continuation loop. Runs on
    // the truncated-non-empty case; empty + exhausted retries falls
    // through with empty combined and final_complete=false.
    for _ in 0..opts.max_continuations {
        if is_structurally_complete(&combined, opts.shape) {
            return Ok(ContinuableOutcome {
                text: combined,
                empty_retries,
                continuations,
                final_complete: true,
            });
        }
        if combined.trim().is_empty() {
            // Nothing to continue from — continuing "" is identical to
            // the initial call and would loop. Bail so the caller sees
            // the failure rather than burning N extra calls.
            break;
        }
        let cont_prompt = continuation_prompt(prompt, &combined);
        let req = make_request(opts, cont_prompt, current_max.min(opts.budget_cap));
        let resp = generator.generate_text(req).await?;
        combined.push_str(&resp.text);
        continuations += 1;
    }

    let final_complete = is_structurally_complete(&combined, opts.shape);
    Ok(ContinuableOutcome {
        text: combined,
        empty_retries,
        continuations,
        final_complete,
    })
}

/// Scripted generator for unit tests. Returns responses from `script`
/// in order; extra calls reuse the last entry so tests don't have to
/// count past what they actually assert on.
#[cfg(test)]
pub struct ScriptedGenerator {
    script: Vec<Result<String, String>>,
    calls: std::sync::Arc<std::sync::Mutex<Vec<GenerateRequest>>>,
}

#[cfg(test)]
impl ScriptedGenerator {
    pub fn new<I>(script: I) -> Self
    where
        I: IntoIterator<Item = Result<String, String>>,
    {
        Self {
            script: script.into_iter().collect(),
            calls: std::sync::Arc::new(std::sync::Mutex::new(Vec::new())),
        }
    }

    pub fn calls(&self) -> Vec<GenerateRequest> {
        self.calls.lock().unwrap().clone()
    }

    pub fn call_count(&self) -> usize {
        self.calls.lock().unwrap().len()
    }
}

#[cfg(test)]
impl TextGenerator for ScriptedGenerator {
    fn generate_text(
        &self,
        req: GenerateRequest,
    ) -> impl Future<Output = Result<GenerateResponse, String>> + Send {
        let i = {
            let mut calls = self.calls.lock().unwrap();
            let i = calls.len();
            calls.push(req.clone());
            i
        };
        let model = req.model.clone().unwrap_or_default();
        let entry = self.script.get(i)
            .cloned()
            .unwrap_or_else(|| self.script.last().cloned().unwrap_or(Ok(String::new())));
        async move {
            entry.map(|text| GenerateResponse {
                text,
                model,
                tokens_evaluated: None,
                tokens_generated: None,
            })
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn structural_complete_rejects_empty_and_text_mismatch() {
        assert!(!is_structurally_complete("", ResponseShape::Text));
        assert!(!is_structurally_complete("   ", ResponseShape::Text));
        assert!(is_structurally_complete("any content", ResponseShape::Text));
    }

    #[test]
    fn structural_complete_handles_balanced_json() {
        assert!(is_structurally_complete(r#"{"a": 1}"#, ResponseShape::Json));
        assert!(is_structurally_complete(
            r#"```json
{"a": 1, "b": [1, 2, 3]}
```"#,
            ResponseShape::Json,
        ));
    }

    #[test]
    fn structural_complete_rejects_truncated_json() {
        assert!(!is_structurally_complete(r#"{"a": 1"#, ResponseShape::Json));
        assert!(!is_structurally_complete(r#"{"a": {"b": 1"#, ResponseShape::Json));
        // Trailing comma — balanced but unparseable
        assert!(!is_structurally_complete(r#"{"a": 1,}"#, ResponseShape::Json));
    }

    #[test]
    fn structural_complete_ignores_braces_inside_strings() {
        assert!(is_structurally_complete(r#"{"s": "has } inside"}"#, ResponseShape::Json));
        assert!(is_structurally_complete(r#"{"s": "escaped \" quote"}"#, ResponseShape::Json));
    }

    #[tokio::test]
    async fn continuable_returns_first_response_when_complete() {
        let generator = ScriptedGenerator::new(vec![Ok(r#"{"ok": true}"#.to_string())]);
        let opts = ContinuableOpts::new("qwen3:latest");
        let out = generate_continuable(&generator, "test", &opts).await.unwrap();
        assert!(out.final_complete);
        assert_eq!(out.empty_retries, 0);
        assert_eq!(out.continuations, 0);
        assert_eq!(generator.call_count(), 1);
    }

    #[tokio::test]
    async fn continuable_retries_on_empty_with_doubled_budget() {
        // Two empties, then a good response. Third call should see 4×
        // the initial budget (2× twice).
        let generator = ScriptedGenerator::new(vec![
            Ok("".to_string()),
            Ok("".to_string()),
            Ok(r#"{"ok": true}"#.to_string()),
        ]);
        let mut opts = ContinuableOpts::new("qwen3:latest");
        opts.max_tokens = Some(100);
        let out = generate_continuable(&generator, "test", &opts).await.unwrap();
        assert!(out.final_complete);
        assert_eq!(out.empty_retries, 2);

        let calls = generator.calls();
        assert_eq!(calls.len(), 3);
        assert_eq!(calls[0].max_tokens, Some(100));
        assert_eq!(calls[1].max_tokens, Some(200));
        assert_eq!(calls[2].max_tokens, Some(400));
    }

    #[tokio::test]
    async fn continuable_caps_budget_at_budget_cap() {
        let generator = ScriptedGenerator::new(vec![
            Ok("".to_string()),
            Ok("".to_string()),
            Ok(r#"{"ok":1}"#.to_string()),
        ]);
        let mut opts = ContinuableOpts::new("qwen3:latest");
        opts.max_tokens = Some(5_000);
        opts.budget_cap = 7_000;
        let out = generate_continuable(&generator, "test", &opts).await.unwrap();
        assert!(out.final_complete);
        let calls = generator.calls();
        // 5000 → doubled would be 10000; cap pulls it to 7000.
        assert_eq!(calls[1].max_tokens, Some(7_000));
        assert_eq!(calls[2].max_tokens, Some(7_000));
    }

    #[tokio::test]
    async fn continuable_glues_truncated_response() {
        // First call returns balanced-open `{...`; continuation closes
        // it with `...}`. Combined must parse.
        let generator = ScriptedGenerator::new(vec![
            Ok(r#"{"fills": [{"candidate_id": "C-001""#.to_string()),
            Ok(r#", "name": "Alice"}]}"#.to_string()),
        ]);
        let opts = ContinuableOpts::new("qwen3:latest");
        let out = generate_continuable(&generator, "ORIGINAL", &opts).await.unwrap();
        assert!(out.final_complete, "combined must parse: {}", out.text);
        assert_eq!(out.continuations, 1);

        let calls = generator.calls();
        assert_eq!(calls.len(), 2);
        // Continuation prompt must contain the partial — that's the
        // scratchpad primitive J called out.
        let cont_prompt = &calls[1].prompt;
        assert!(cont_prompt.contains("ORIGINAL"),
            "continuation must include original prompt");
        assert!(cont_prompt.contains(r#"{"fills": [{"candidate_id": "C-001""#),
            "continuation must include partial");
    }

    #[tokio::test]
    async fn continuable_does_not_loop_on_persistent_empty() {
        // All three retries return empty; we must NOT then enter the
        // continuation loop with an empty partial (would burn 3 more
        // calls continuing from "").
        let generator = ScriptedGenerator::new(vec![
            Ok("".to_string()),
            Ok("".to_string()),
            Ok("".to_string()),
        ]);
        let opts = ContinuableOpts::new("qwen3:latest");
        let out = generate_continuable(&generator, "test", &opts).await.unwrap();
        assert!(!out.final_complete);
        assert_eq!(out.empty_retries, 3);
        assert_eq!(out.continuations, 0, "must not continue from empty");
        assert_eq!(generator.call_count(), 3);
    }

    #[tokio::test]
    async fn continuable_returns_raw_on_exhausted_continuations() {
        // Three continuations that never complete — caller's parser
        // will throw. We must return the combined text so forensics
        // has the raw content, not a lossy truncation.
        let generator = ScriptedGenerator::new(vec![
            Ok(r#"{"a": ["#.to_string()),
            Ok("1,".to_string()),
            Ok("2,".to_string()),
            Ok("3,".to_string()),
        ]);
        let mut opts = ContinuableOpts::new("qwen3:latest");
        opts.max_continuations = 3;
        let out = generate_continuable(&generator, "test", &opts).await.unwrap();
        assert!(!out.final_complete);
        assert_eq!(out.continuations, 3);
        assert!(out.text.contains(r#"{"a": ["#));
        assert!(out.text.contains("3,"));
    }

    #[tokio::test]
    async fn continuable_propagates_generator_errors() {
        let generator = ScriptedGenerator::new(vec![Err("sidecar 503".to_string())]);
        let opts = ContinuableOpts::new("qwen3:latest");
        let err = generate_continuable(&generator, "test", &opts).await.unwrap_err();
        assert!(err.contains("503"));
    }
}