From 7e2406cc77bfe55e898342cbd267061d45e67f7f Mon Sep 17 00:00:00 2001
From: Jack Grigg <jack@electriccoin.co>
Date: Fri, 8 Jan 2021 01:54:28 +0000
Subject: [PATCH] Implement a simple single-chip layouter

---
 src/circuit/layouter.rs | 160 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 158 insertions(+), 2 deletions(-)

diff --git a/src/circuit/layouter.rs b/src/circuit/layouter.rs
index 9e226af8..fa329f5b 100644
--- a/src/circuit/layouter.rs
+++ b/src/circuit/layouter.rs
@@ -1,9 +1,11 @@
 //! Implementations of common circuit layouters.
 
+use std::cmp;
 use std::fmt;
+use std::marker::PhantomData;
 
-use super::{Cell, Chip, Permutation};
-use crate::plonk::{Advice, Column, Error, Fixed};
+use super::{Cell, Chip, Layouter, Permutation, Region};
+use crate::plonk::{Advice, Assignment, Column, Error, Fixed};
 
 /// Helper trait for implementing a custom [`Layouter`].
 ///
@@ -60,3 +62,157 @@ pub trait RegionLayouter<C: Chip>: fmt::Debug {
         right: Cell,
     ) -> Result<(), Error>;
 }
+
+/// A [`Layouter`] for a single-chip circuit.
+pub struct SingleChip<'a, C: Chip, CS: Assignment<C::Field> + 'a> {
+    cs: &'a mut CS,
+    config: C::Config,
+    regions: Vec<usize>,
+    current_gate: usize,
+    _marker: PhantomData<C>,
+}
+
+impl<'a, C: Chip, CS: Assignment<C::Field> + 'a> fmt::Debug for SingleChip<'a, C, CS> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("SingleChip")
+            .field("config", &self.config)
+            .field("regions", &self.regions)
+            .field("current_gate", &self.current_gate)
+            .finish()
+    }
+}
+
+impl<'a, C: Chip, CS: Assignment<C::Field>> SingleChip<'a, C, CS> {
+    /// Creates a new single-chip layouter.
+    pub fn new(cs: &'a mut CS, config: C::Config) -> Self {
+        SingleChip {
+            cs,
+            config,
+            regions: vec![],
+            current_gate: 0,
+            _marker: PhantomData,
+        }
+    }
+}
+
+impl<'a, C: Chip, CS: Assignment<C::Field> + 'a> Layouter<C> for SingleChip<'a, C, CS> {
+    fn config(&self) -> &C::Config {
+        &self.config
+    }
+
+    fn assign_region(
+        &mut self,
+        assignment: impl FnOnce(Region<'_, C>) -> Result<(), Error>,
+    ) -> Result<(), Error> {
+        let region_index = self.regions.len();
+        self.regions.push(self.current_gate);
+
+        let mut region = SingleChipRegion::new(self, region_index);
+        {
+            let region: &mut dyn RegionLayouter<C> = &mut region;
+            assignment(region.into())?;
+        }
+        self.current_gate += region.row_count;
+
+        Ok(())
+    }
+}
+
+struct SingleChipRegion<'r, 'a, C: Chip, CS: Assignment<C::Field> + 'a> {
+    layouter: &'r mut SingleChip<'a, C, CS>,
+    region_index: usize,
+    row_count: usize,
+}
+
+impl<'r, 'a, C: Chip, CS: Assignment<C::Field> + 'a> fmt::Debug
+    for SingleChipRegion<'r, 'a, C, CS>
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("SingleChipRegion")
+            .field("layouter", &self.layouter)
+            .field("region_index", &self.region_index)
+            .field("row_count", &self.row_count)
+            .finish()
+    }
+}
+
+impl<'r, 'a, C: Chip, CS: Assignment<C::Field> + 'a> SingleChipRegion<'r, 'a, C, CS> {
+    fn new(layouter: &'r mut SingleChip<'a, C, CS>, region_index: usize) -> Self {
+        SingleChipRegion {
+            layouter,
+            region_index,
+            row_count: 0,
+        }
+    }
+}
+
+impl<'r, 'a, C: Chip, CS: Assignment<C::Field> + 'a> RegionLayouter<C>
+    for SingleChipRegion<'r, 'a, C, CS>
+{
+    fn assign_advice<'v>(
+        &'v mut self,
+        column: Column<Advice>,
+        offset: usize,
+        to: &'v mut (dyn FnMut() -> Result<C::Field, Error> + 'v),
+    ) -> Result<Cell, Error> {
+        self.layouter.cs.assign_advice(
+            column,
+            self.layouter.regions[self.region_index] + offset,
+            to,
+        )?;
+        self.row_count = cmp::max(self.row_count, offset);
+
+        Ok(Cell {
+            region_index: self.region_index,
+            row_offset: offset,
+            column: column.into(),
+        })
+    }
+
+    fn assign_fixed<'v>(
+        &'v mut self,
+        column: Column<Fixed>,
+        offset: usize,
+        to: &'v mut (dyn FnMut() -> Result<C::Field, Error> + 'v),
+    ) -> Result<Cell, Error> {
+        self.layouter.cs.assign_fixed(
+            column,
+            self.layouter.regions[self.region_index] + offset,
+            to,
+        )?;
+        self.row_count = cmp::max(self.row_count, offset);
+        Ok(Cell {
+            region_index: self.region_index,
+            row_offset: offset,
+            column: column.into(),
+        })
+    }
+
+    fn constrain_equal(
+        &mut self,
+        permutation: &Permutation,
+        left: Cell,
+        right: Cell,
+    ) -> Result<(), Error> {
+        let left_column = permutation
+            .mapping
+            .iter()
+            .position(|c| c == &left.column)
+            .ok_or(Error::SynthesisError)?;
+        let right_column = permutation
+            .mapping
+            .iter()
+            .position(|c| c == &right.column)
+            .ok_or(Error::SynthesisError)?;
+
+        self.layouter.cs.copy(
+            permutation.index,
+            left_column,
+            self.layouter.regions[left.region_index] + left.row_offset,
+            right_column,
+            self.layouter.regions[right.region_index] + right.row_offset,
+        )?;
+
+        Ok(())
+    }
+}