From 4d8ae89aa9331c3f0d24e183951b725bf03ab4b0 Mon Sep 17 00:00:00 2001
From: therealyingtong <yingtong@z.cash>
Date: Thu, 3 Jun 2021 09:41:03 +0800
Subject: [PATCH] Add Utilities chip

---
 src/circuit/gadget.rs           |  1 +
 src/circuit/gadget/utilities.rs | 78 +++++++++++++++++++++++++++++++++
 2 files changed, 79 insertions(+)
 create mode 100644 src/circuit/gadget/utilities.rs

diff --git a/src/circuit/gadget.rs b/src/circuit/gadget.rs
index 7d6426a7..20572cd7 100644
--- a/src/circuit/gadget.rs
+++ b/src/circuit/gadget.rs
@@ -1,2 +1,3 @@
 pub(crate) mod ecc;
 pub(crate) mod poseidon;
+pub(crate) mod utilities;
diff --git a/src/circuit/gadget/utilities.rs b/src/circuit/gadget/utilities.rs
new file mode 100644
index 00000000..fe8d6925
--- /dev/null
+++ b/src/circuit/gadget/utilities.rs
@@ -0,0 +1,78 @@
+use halo2::{
+    circuit::{Cell, Chip, Layouter, Region},
+    plonk::{Advice, Column, Error, Permutation},
+};
+use pasta_curves::arithmetic::FieldExt;
+
+/// A variable representing a number.
+#[derive(Copy, Clone, Debug)]
+pub struct CellValue<F: FieldExt> {
+    cell: Cell,
+    value: Option<F>,
+}
+
+pub trait Var<F: FieldExt> {
+    fn new(cell: Cell, value: Option<F>) -> Self;
+    fn cell(&self) -> Cell;
+    fn value(&self) -> Option<F>;
+}
+
+impl<F: FieldExt> Var<F> for CellValue<F> {
+    fn new(cell: Cell, value: Option<F>) -> Self {
+        Self { cell, value }
+    }
+
+    fn cell(&self) -> Cell {
+        self.cell
+    }
+
+    fn value(&self) -> Option<F> {
+        self.value
+    }
+}
+
+pub trait UtilitiesInstructions<F: FieldExt>: Chip<F> {
+    type Var: Var<F>;
+
+    fn load_private(
+        &self,
+        mut layouter: impl Layouter<F>,
+        column: Column<Advice>,
+        value: Option<F>,
+    ) -> Result<Self::Var, Error> {
+        layouter.assign_region(
+            || "load private",
+            |mut region| {
+                let cell = region.assign_advice(
+                    || "load private",
+                    column,
+                    0,
+                    || value.ok_or(Error::SynthesisError),
+                )?;
+                Ok(Var::new(cell, value))
+            },
+        )
+    }
+}
+
+/// Assign a cell the same value as another cell and set up a copy constraint between them.
+pub fn copy<A, AR, F: FieldExt>(
+    region: &mut Region<'_, F>,
+    annotation: A,
+    column: Column<Advice>,
+    row: usize,
+    copy: &CellValue<F>,
+    perm: &Permutation,
+) -> Result<CellValue<F>, Error>
+where
+    A: Fn() -> AR,
+    AR: Into<String>,
+{
+    let cell = region.assign_advice(annotation, column, row, || {
+        copy.value.ok_or(Error::SynthesisError)
+    })?;
+
+    region.constrain_equal(perm, cell, copy.cell)?;
+
+    Ok(CellValue::new(cell, copy.value))
+}