dev: Add halo2::dev::render_to_json API

This exposes the internally-collected data used by CircuitLayout, to
enable alternative external visualisations.

halo2_proofs/Cargo.toml

@@ -55,6 +55,8 @@ maybe-rayon = {version = "0.1.0", default-features = false}
 
 # Developer tooling dependencies
 plotters = { version = "0.3.0", default-features = false, optional = true }
+serde = { version = "1", features = ["derive"], optional = true }
+serde_json = { version = "1", optional = true }
 tabbycat = { version = "0.1", features = ["attributes"], optional = true }
 
 # Legacy circuit compatibility
@@ -73,7 +75,7 @@ getrandom = { version = "0.2", features = ["js"] }
 [features]
 default = ["batch", "multicore"]
 multicore = ["maybe-rayon/threads"]
-dev-graph = ["plotters", "tabbycat"]
+dev-graph = ["plotters", "serde", "serde_json", "tabbycat"]
 test-dev-graph = [
     "dev-graph",
     "plotters/bitmap_backend",

halo2_proofs/src/circuit/layouter.rs

@@ -139,6 +139,7 @@ pub struct RegionShape {
 /// The virtual column involved in a region. This includes concrete columns,
 /// as well as selectors that are not concrete columns at this stage.
 #[derive(Eq, PartialEq, Copy, Clone, Debug, Hash)]
+#[cfg_attr(feature = "dev-graph", derive(serde::Serialize))]
 pub enum RegionColumn {
     /// Concrete column
     Column(Column<Any>),

halo2_proofs/src/dev.rs

@@ -36,7 +36,10 @@ mod graph;
 
 #[cfg(feature = "dev-graph")]
 #[cfg_attr(docsrs, doc(cfg(feature = "dev-graph")))]
-pub use graph::{circuit_dot_graph, layout::CircuitLayout};
+pub use graph::{
+    circuit_dot_graph,
+    layout::{render_to_json, CircuitLayout},
+};
 
 #[derive(Debug)]
 struct Region {

halo2_proofs/src/dev/cost.rs

@@ -11,6 +11,9 @@ use std::{
 use ff::{Field, PrimeField};
 use group::prime::PrimeGroup;
 
+#[cfg(feature = "dev-graph")]
+use serde::ser::SerializeStruct;
+
 use crate::{
     circuit::{layouter::RegionColumn, Value},
     plonk::{
@@ -54,7 +57,28 @@ pub struct CircuitCost<G: PrimeGroup, ConcreteCircuit: Circuit<G::Scalar>> {
     _marker: PhantomData<(G, ConcreteCircuit)>,
 }
 
+#[cfg(feature = "dev-graph")]
+impl serde::Serialize for Column<Any> {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        let mut state = serializer.serialize_struct("Column", 2)?;
+        state.serialize_field(
+            "kind",
+            match self.column_type() {
+                Any::Advice => "advice",
+                Any::Fixed => "fixed",
+                Any::Instance => "instance",
+            },
+        )?;
+        state.serialize_field("index", &self.index())?;
+        state.end()
+    }
+}
+
 #[derive(Debug)]
+#[cfg_attr(feature = "dev-graph", derive(serde::Serialize))]
 pub(crate) struct Cell {
     pub(crate) column: RegionColumn,
     pub(crate) row: usize,
@@ -63,6 +87,7 @@ pub(crate) struct Cell {
 /// Region implementation used by Layout
 #[allow(dead_code)]
 #[derive(Debug)]
+#[cfg_attr(feature = "dev-graph", derive(serde::Serialize))]
 pub(crate) struct LayoutRegion {
     /// The name of the region. Not required to be unique.
     pub(crate) name: String,

halo2_proofs/src/dev/graph/layout.rs

@@ -3,12 +3,13 @@ use plotters::{
     coord::Shift,
     prelude::{DrawingArea, DrawingAreaErrorKind, DrawingBackend},
 };
+
 use std::collections::HashSet;
 use std::ops::Range;
 
 use crate::{
     circuit::layouter::RegionColumn,
-    dev::cost::{Cell, Layout},
+    dev::cost::{Cell, Layout, LayoutRegion},
     plonk::{Any, Circuit, Column, ConstraintSystem, FloorPlanner},
 };
 
@@ -318,3 +319,35 @@ impl CircuitLayout {
         Ok(())
     }
 }
+
+/// Renders the given circuit layout to a JSON string.
+pub fn render_to_json<F: Field, ConcreteCircuit: Circuit<F>>(
+    circuit: &ConcreteCircuit,
+) -> Result<String, serde_json::Error> {
+    // Collect the layout details.
+    let mut cs = ConstraintSystem::default();
+    let config = ConcreteCircuit::configure(&mut cs);
+    let mut layout = Layout::default();
+    ConcreteCircuit::FloorPlanner::synthesize(&mut layout, circuit, config, cs.constants).unwrap();
+
+    // Render.
+    #[derive(serde::Serialize)]
+    struct Circuit {
+        num_instance_columns: usize,
+        num_advice_columns: usize,
+        num_fixed_columns: usize,
+        total_rows: usize,
+        regions: Vec<LayoutRegion>,
+        loose_cells: Vec<Cell>,
+        selectors: Vec<Vec<bool>>,
+    }
+    serde_json::to_string(&Circuit {
+        num_instance_columns: cs.num_instance_columns,
+        num_advice_columns: cs.num_advice_columns,
+        num_fixed_columns: cs.num_fixed_columns,
+        total_rows: layout.total_rows,
+        regions: layout.regions,
+        loose_cells: layout.loose_cells,
+        selectors: layout.selectors,
+    })
+}

halo2_proofs/src/plonk/circuit.rs

@@ -253,6 +253,7 @@ impl TryFrom<Column<Any>> for Column<Instance> {
 /// }
 /// ```
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+#[cfg_attr(feature = "dev-graph", derive(serde::Serialize))]
 pub struct Selector(pub(crate) usize, bool);
 
 impl Selector {