Fix #1059: add pred extract for lifting external target-space solutions

problemreductions-cli/src/cli.rs

@@ -159,6 +159,21 @@ Examples:
     Inspect(InspectArgs),
     /// Solve a problem instance
     Solve(SolveArgs),
+    /// Extract a source-space solution from a reduction bundle and a target-space config
+    #[command(after_help = "\
+Examples:
+  pred extract bundle.json --config 1,0,1,0
+  pred extract bundle.json --config 1,0,1,0 -o source.json
+  cat bundle.json | pred extract - --config 1,0,1,0
+
+Use this when an external solver has solved the bundle's target problem
+(e.g. a QUBO sampler, a neutral-atom platform, a QAOA runtime) and you want
+the corresponding solution in the original source problem space without
+having to shell back into `pred solve`.
+
+Input: a reduction bundle JSON (from `pred reduce`). Use - to read from stdin.
+--config is the target-space configuration (comma-separated, e.g. 1,0,1,0).")]
+    Extract(ExtractArgs),
     /// Start MCP (Model Context Protocol) server for AI assistant integration
     #[cfg(feature = "mcp")]
     #[command(after_help = "\
@@ -1209,6 +1224,15 @@ pub struct ReduceArgs {
     pub via: Option<PathBuf>,
 }
 
+#[derive(clap::Args)]
+pub struct ExtractArgs {
+    /// Reduction bundle JSON (from `pred reduce`). Use - for stdin.
+    pub input: PathBuf,
+    /// Target-space configuration to map back (comma-separated, e.g. 1,0,1,0)
+    #[arg(long)]
+    pub config: String,
+}
+
 #[derive(clap::Args)]
 pub struct InspectArgs {
     /// Problem JSON file or reduction bundle. Use - for stdin.
@@ -1242,6 +1266,7 @@ pub fn print_subcommand_help_hint(error_msg: &str) {
     let subcmds = [
         ("pred solve", "solve"),
         ("pred reduce", "reduce"),
+        ("pred extract", "extract"),
         ("pred create", "create"),
         ("pred evaluate", "evaluate"),
         ("pred inspect", "inspect"),

problemreductions-cli/src/commands/extract.rs

@@ -0,0 +1,87 @@
+use crate::dispatch::{read_input, BundleReplay, ReductionBundle};
+use crate::output::OutputConfig;
+use anyhow::{Context, Result};
+use std::path::Path;
+
+/// Extract a source-space configuration from a target-space configuration and a reduction bundle.
+///
+/// This lets external solvers (that solved the bundle's target problem on their own)
+/// recover a solution in the original source problem space without having to
+/// re-solve through `pred solve`.
+pub fn extract(input: &Path, config_str: &str, out: &OutputConfig) -> Result<()> {
+    let content = read_input(input)?;
+    let json: serde_json::Value =
+        serde_json::from_str(&content).context("Input is not valid JSON")?;
+
+    if !(json.get("source").is_some() && json.get("target").is_some() && json.get("path").is_some())
+    {
+        anyhow::bail!(
+            "Input is not a reduction bundle.\n\
+             `pred extract` requires a bundle produced by `pred reduce`.\n\
+             Got a plain problem file; did you mean `pred evaluate`?"
+        );
+    }
+
+    let bundle: ReductionBundle =
+        serde_json::from_value(json).context("Failed to parse reduction bundle")?;
+
+    // An empty --config means an empty target configuration (zero-variable target problem).
+    let target_config: Vec<usize> = if config_str.trim().is_empty() {
+        Vec::new()
+    } else {
+        config_str
+            .split(',')
+            .map(|s| {
+                s.trim()
+                    .parse::<usize>()
+                    .map_err(|e| anyhow::anyhow!("Invalid config value '{}': {}", s.trim(), e))
+            })
+            .collect::<Result<Vec<_>>>()?
+    };
+
+    let replay = BundleReplay::prepare(&bundle)?;
+
+    let target_dims = replay.target.dims_dyn();
+    if target_config.len() != target_dims.len() {
+        anyhow::bail!(
+            "Target config has {} values but target problem {} has {} variables",
+            target_config.len(),
+            replay.target_name,
+            target_dims.len()
+        );
+    }
+    for (i, (val, dim)) in target_config.iter().zip(target_dims.iter()).enumerate() {
+        if *val >= *dim {
+            anyhow::bail!(
+                "Target config value {} at position {} is out of range: variable {} has {} possible values (0..{})",
+                val, i, i, dim, dim.saturating_sub(1)
+            );
+        }
+    }
+    let target_eval = replay.target.evaluate_dyn(&target_config);
+
+    let (source_config, source_eval) = replay.extract(&target_config);
+
+    let text = format!(
+        "Problem: {}\nSolver: external (via {})\nSolution: {:?}\nEvaluation: {}",
+        replay.source_name, replay.target_name, source_config, source_eval,
+    );
+
+    // Schema aligned with `pred solve` on a bundle: `problem`, `reduced_to`, `solution`,
+    // `evaluation`, `intermediate { problem, solution, evaluation }`. `solver` is "external"
+    // to signal that pred did not run a solver — the target config came from outside.
+    let json = serde_json::json!({
+        "problem": replay.source_name,
+        "solver": "external",
+        "reduced_to": replay.target_name,
+        "solution": source_config,
+        "evaluation": source_eval,
+        "intermediate": {
+            "problem": replay.target_name,
+            "solution": target_config,
+            "evaluation": target_eval,
+        },
+    });
+
+    out.emit_with_default_name("pred_extract.json", &text, &json)
+}

problemreductions-cli/src/commands/mod.rs

@@ -1,5 +1,6 @@
 pub mod create;
 pub mod evaluate;
+pub mod extract;
 pub mod graph;
 pub mod inspect;
 pub mod reduce;

problemreductions-cli/src/commands/solve.rs

@@ -1,7 +1,6 @@
-use crate::dispatch::{load_problem, read_input, ProblemJson, ReductionBundle};
+use crate::dispatch::{load_problem, read_input, BundleReplay, ProblemJson, ReductionBundle};
 use crate::output::OutputConfig;
 use anyhow::{Context, Result};
-use problemreductions::rules::ReductionGraph;
 use std::path::Path;
 use std::time::Duration;
 
@@ -166,75 +165,39 @@ fn solve_problem(
 
 /// Solve a reduction bundle: solve the target problem, then map the solution back.
 fn solve_bundle(bundle: ReductionBundle, solver_name: &str, out: &OutputConfig) -> Result<()> {
-    // 1. Load the target problem from the bundle
-    let target = load_problem(
-        &bundle.target.problem_type,
-        &bundle.target.variant,
-        bundle.target.data.clone(),
-    )?;
-    let target_name = target.problem_name();
+    let replay = BundleReplay::prepare(&bundle)?;
 
-    // 2. Solve the target problem
     let target_result = match solver_name {
-        "brute-force" => target.solve_brute_force_witness().ok_or_else(|| {
+        "brute-force" => replay.target.solve_brute_force_witness().ok_or_else(|| {
             anyhow::anyhow!(
                 "Bundle solving requires a witness-capable target problem and witness-capable reduction path; {} only supports aggregate-value solving.",
-                target_name
+                replay.target_name
             )
         })?,
-        "ilp" => target.solve_with_ilp().map_err(add_ilp_solver_hint)?,
-        "customized" => target
+        "ilp" => replay.target.solve_with_ilp().map_err(add_ilp_solver_hint)?,
+        "customized" => replay
+            .target
             .solve_with_customized()
             .map_err(add_customized_solver_hint)?,
         _ => unreachable!(),
     };
 
-    // 3. Load source problem and re-execute the reduction chain to get extract_solution
-    let source = load_problem(
-        &bundle.source.problem_type,
-        &bundle.source.variant,
-        bundle.source.data.clone(),
-    )?;
-    let source_name = source.problem_name();
+    let (source_config, source_eval) = replay.extract(&target_result.config);
 
-    let graph = ReductionGraph::new();
-
-    // Reconstruct the ReductionPath from the bundle's path steps
-    let reduction_path = problemreductions::rules::ReductionPath {
-        steps: bundle
-            .path
-            .iter()
-            .map(|s| problemreductions::rules::ReductionStep {
-                name: s.name.clone(),
-                variant: s.variant.clone(),
-            })
-            .collect(),
-    };
-
-    let chain = graph
-        .reduce_along_path(&reduction_path, source.as_any())
-        .ok_or_else(|| anyhow::anyhow!(
-            "Bundle solving requires a witness-capable reduction path; this bundle cannot recover a source solution."
-        ))?;
-
-    // 4. Extract solution back to source problem space
-    let source_config = chain.extract_solution(&target_result.config);
-    let source_eval = source.evaluate_dyn(&source_config);
-
-    let solver_desc = format!("{} (via {})", solver_name, target_name);
+    let solver_desc = format!("{} (via {})", solver_name, replay.target_name);
     let text = format!(
         "Problem: {}\nSolver: {}\nSolution: {:?}\nEvaluation: {}",
-        source_name, solver_desc, source_config, source_eval,
+        replay.source_name, solver_desc, source_config, source_eval,
     );
 
     let json = serde_json::json!({
-        "problem": source_name,
+        "problem": replay.source_name,
         "solver": solver_name,
-        "reduced_to": target_name,
+        "reduced_to": replay.target_name,
         "solution": source_config,
         "evaluation": source_eval,
         "intermediate": {
-            "problem": target_name,
+            "problem": replay.target_name,
             "solution": target_result.config,
             "evaluation": target_result.evaluation,
         },

problemreductions-cli/src/dispatch.rs

@@ -114,6 +114,130 @@ impl LoadedProblem {
     }
 }
 
+/// A validated reduction bundle ready to replay:
+/// source, target, and the reconstructed reduction chain. Construct via
+/// [`BundleReplay::prepare`]. All three CLI/MCP bundle workflows
+/// (`pred solve <bundle>`, `pred extract <bundle>`, MCP `solve_problem`)
+/// share this setup so validation and error text stay in sync.
+pub struct BundleReplay {
+    pub(crate) source: LoadedProblem,
+    pub(crate) source_name: String,
+    pub(crate) target: LoadedProblem,
+    pub(crate) target_name: String,
+    pub(crate) chain: problemreductions::rules::ReductionChain,
+}
+
+impl BundleReplay {
+    /// Validate the bundle and replay the reduction chain.
+    ///
+    /// Checks:
+    /// - `path` has at least two steps
+    /// - `path[0]` matches `source` (name + variant)
+    /// - `path[-1]` matches `target` (name + variant)
+    /// - serializing the chain's replayed target equals `bundle.target.data`
+    ///   (tampered/stale bundles where `target.data` disagrees with what
+    ///   `reduce_along_path` actually produced are rejected)
+    ///
+    /// Returns an error (not a panic) for malformed bundles or aggregate-only paths.
+    pub fn prepare(bundle: &ReductionBundle) -> Result<Self> {
+        if bundle.path.len() < 2 {
+            anyhow::bail!(
+                "Malformed bundle: `path` must contain at least two steps (source and target), got {}",
+                bundle.path.len()
+            );
+        }
+        let first = bundle.path.first().unwrap();
+        let last = bundle.path.last().unwrap();
+        if first.name != bundle.source.problem_type || first.variant != bundle.source.variant {
+            anyhow::bail!(
+                "Malformed bundle: path starts with {} but source is {}",
+                format_step(&first.name, &first.variant),
+                format_step(&bundle.source.problem_type, &bundle.source.variant),
+            );
+        }
+        if last.name != bundle.target.problem_type || last.variant != bundle.target.variant {
+            anyhow::bail!(
+                "Malformed bundle: path ends with {} but target is {}",
+                format_step(&last.name, &last.variant),
+                format_step(&bundle.target.problem_type, &bundle.target.variant),
+            );
+        }
+
+        let source = load_problem(
+            &bundle.source.problem_type,
+            &bundle.source.variant,
+            bundle.source.data.clone(),
+        )?;
+        let source_name = source.problem_name().to_string();
+
+        let target = load_problem(
+            &bundle.target.problem_type,
+            &bundle.target.variant,
+            bundle.target.data.clone(),
+        )?;
+        let target_name = target.problem_name().to_string();
+
+        let reduction_path = problemreductions::rules::ReductionPath {
+            steps: bundle
+                .path
+                .iter()
+                .map(|s| problemreductions::rules::ReductionStep {
+                    name: s.name.clone(),
+                    variant: s.variant.clone(),
+                })
+                .collect(),
+        };
+
+        let graph = ReductionGraph::new();
+        let chain = graph
+            .reduce_along_path(&reduction_path, source.as_any())
+            .ok_or_else(|| anyhow::anyhow!(
+                "Bundle requires a witness-capable reduction path; this bundle cannot map a target solution back to the source."
+            ))?;
+
+        // Coherence check: `bundle.target.data` must equal what replaying
+        // `source` along `path` actually produces. Without this, a caller
+        // could solve/validate against the bundle's stated target but then
+        // extract through a completely different chain target.
+        let replayed_target_data =
+            serialize_any_problem(&last.name, &last.variant, chain.target_problem_any())?;
+        if replayed_target_data != bundle.target.data {
+            anyhow::bail!(
+                "Malformed bundle: `target.data` does not match the result of replaying \
+                 `source` along `path`. The bundle is tampered or was produced by \
+                 incompatible code."
+            );
+        }
+
+        Ok(Self {
+            source,
+            source_name,
+            target,
+            target_name,
+            chain,
+        })
+    }
+
+    /// Map a target-space configuration back to the source space and evaluate it.
+    pub fn extract(&self, target_config: &[usize]) -> (Vec<usize>, String) {
+        let source_config = self.chain.extract_solution(target_config);
+        let source_eval = self.source.evaluate_dyn(&source_config);
+        (source_config, source_eval)
+    }
+}
+
+fn format_step(name: &str, variant: &BTreeMap<String, String>) -> String {
+    if variant.is_empty() {
+        name.to_string()
+    } else {
+        let parts: Vec<String> = variant
+            .iter()
+            .map(|(k, v)| format!("{}={}", k, v))
+            .collect();
+        format!("{}{{{}}}", name, parts.join(", "))
+    }
+}
+
 /// Load a problem from JSON type/variant/data.
 pub fn load_problem(
     name: &str,

problemreductions-cli/src/main.rs

@@ -34,7 +34,11 @@ fn main() -> anyhow::Result<()> {
     // Data-producing commands auto-output JSON when piped
     let auto_json = matches!(
         cli.command,
-        Commands::Reduce(_) | Commands::Solve(_) | Commands::Evaluate(_) | Commands::Inspect(_)
+        Commands::Reduce(_)
+            | Commands::Solve(_)
+            | Commands::Evaluate(_)
+            | Commands::Inspect(_)
+            | Commands::Extract(_)
     );
 
     let out = OutputConfig {
@@ -72,6 +76,7 @@ fn main() -> anyhow::Result<()> {
             commands::reduce::reduce(&args.input, args.to.as_deref(), args.via.as_deref(), &out)
         }
         Commands::Evaluate(args) => commands::evaluate::evaluate(&args.input, &args.config, &out),
+        Commands::Extract(args) => commands::extract::extract(&args.input, &args.config, &out),
         #[cfg(feature = "mcp")]
         Commands::Mcp => mcp::run(),
         Commands::Completions { shell } => {