Move Sapling verification checks into zcash_proofs crate

zcash_proofs/src/sapling/mod.rs

@@ -0,0 +1,37 @@
+use pairing::bls12_381::Bls12;
+use sapling_crypto::jubjub::{
+    edwards, fs::FsRepr, FixedGenerators, JubjubBls12, JubjubParams, Unknown,
+};
+
+mod verifier;
+
+pub use self::verifier::SaplingVerificationContext;
+
+// This function computes `value` in the exponent of the value commitment base
+pub fn compute_value_balance(
+    value: i64,
+    params: &JubjubBls12,
+) -> Option<edwards::Point<Bls12, Unknown>> {
+    // Compute the absolute value (failing if -i64::MAX is
+    // the value)
+    let abs = match value.checked_abs() {
+        Some(a) => a as u64,
+        None => return None,
+    };
+
+    // Is it negative? We'll have to negate later if so.
+    let is_negative = value.is_negative();
+
+    // Compute it in the exponent
+    let mut value_balance = params
+        .generator(FixedGenerators::ValueCommitmentValue)
+        .mul(FsRepr::from(abs), params);
+
+    // Negate if necessary
+    if is_negative {
+        value_balance = value_balance.negate();
+    }
+
+    // Convert to unknown order point
+    Some(value_balance.into())
+}