Factors out sapling address generation from SaplingExtendedFullViewingKey.

Sapling address generation depends only on an FVK and
a diversifier key. For the purpose of unified full viewing
keys, we need to break the dependence upon Sapling extended
full viewing keys, so this makes the address generation
methods of SaplingExtendedFullViewingKey usable without
coupling them to the ability to derive child full viewing keys.

zcash_primitives/src/zip32.rs

@@ -181,6 +181,42 @@ impl DiversifierKey {
     }
 }
 
+/// Attempt to produce a payment address given the specified diversifier
+/// index, and return None if the specified index does not produce a valid
+/// diversifier.
+pub fn sapling_address(
+    fvk: &FullViewingKey,
+    dk: &DiversifierKey,
+    j: DiversifierIndex,
+) -> Option<PaymentAddress> {
+    dk.diversifier(j)
+        .and_then(|d_j| fvk.vk.to_payment_address(d_j))
+}
+
+/// Search the diversifier space starting at diversifier index `j` for
+/// one which will produce a valid diversifier, and return the payment address
+/// constructed using that diversifier along with the index at which the
+/// valid diversifier was found.
+pub fn sapling_find_address(
+    fvk: &FullViewingKey,
+    dk: &DiversifierKey,
+    j: DiversifierIndex,
+) -> Option<(DiversifierIndex, PaymentAddress)> {
+    let (j, d_j) = dk.find_diversifier(j)?;
+    fvk.vk.to_payment_address(d_j).map(|addr| (j, addr))
+}
+
+/// Returns the payment address corresponding to the smallest valid diversifier
+/// index, along with that index.
+pub fn sapling_default_address(
+    fvk: &FullViewingKey,
+    dk: &DiversifierKey,
+) -> (DiversifierIndex, PaymentAddress) {
+    // This unwrap is safe, if you have to search the 2^88 space of
+    // diversifiers it'll never return anyway.
+    sapling_find_address(fvk, dk, DiversifierIndex::new()).unwrap()
+}
+
 /// A Sapling extended spending key
 #[derive(Clone)]
 pub struct ExtendedSpendingKey {
@@ -445,9 +481,7 @@ impl ExtendedFullViewingKey {
     /// index, and return None if the specified index does not produce a valid
     /// diversifier.
     pub fn address(&self, j: DiversifierIndex) -> Option<PaymentAddress> {
-        self.dk
-            .diversifier(j)
-            .and_then(|d_j| self.fvk.vk.to_payment_address(d_j))
+        sapling_address(&self.fvk, &self.dk, j)
     }
 
     /// Search the diversifier space starting at diversifier index `j` for
@@ -455,16 +489,13 @@ impl ExtendedFullViewingKey {
     /// constructed using that diversifier along with the index at which the
     /// valid diversifier was found.
     pub fn find_address(&self, j: DiversifierIndex) -> Option<(DiversifierIndex, PaymentAddress)> {
-        let (j, d_j) = self.dk.find_diversifier(j)?;
-        self.fvk.vk.to_payment_address(d_j).map(|addr| (j, addr))
+        sapling_find_address(&self.fvk, &self.dk, j)
     }
 
     /// Returns the payment address corresponding to the smallest valid diversifier
     /// index, along with that index.
     pub fn default_address(&self) -> (DiversifierIndex, PaymentAddress) {
-        // This unwrap is safe, if you have to search the 2^88 space of
-        // diversifiers it'll never return anyway.
-        self.find_address(DiversifierIndex::new()).unwrap()
+        sapling_default_address(&self.fvk, &self.dk)
     }
 }