rename the "context string" to the "purpose string"purpose_string

Apart from being pretty ambiguous in general, the term "context string"
has the specific problem that it isn't clear whether it should be
describing the input or the output. In fact, it's quite important that
it describes the output, because the whole point is to domain-separate
different outputs that derive from the *same* input. To make that
clearer, rename the "context string" to the "purpose string" in
documentation.

1 files changed, 7 insertions, 7 deletions

diff --git a/src/test.rs b/src/test.rs
index 7e9fd42..5a4a1ac 100644
--- a/src/test.rs
+++ b/src/test.rs
@@ -319,17 +319,17 @@ fn test_compare_reference_impl() {
 
         // derive_key
         {
-            let context = "BLAKE3 2019-12-27 16:13:59 example context (not the test vector one)";
-            let mut reference_hasher = reference_impl::Hasher::new_derive_key(context);
+            let purpose = "BLAKE3 2019-12-27 16:13:59 example context (not the test vector one)";
+            let mut reference_hasher = reference_impl::Hasher::new_derive_key(purpose);
             reference_hasher.update(input);
             let mut expected_out = [0; OUT];
             reference_hasher.finalize(&mut expected_out);
 
             // all at once
-            let test_out = crate::derive_key(context, input);
+            let test_out = crate::derive_key(purpose, input);
             assert_eq!(test_out[..], expected_out[..32]);
             // incremental
-            let mut hasher = crate::Hasher::new_derive_key(context);
+            let mut hasher = crate::Hasher::new_derive_key(purpose);
             hasher.update(input);
             assert_eq!(hasher.finalize(), *array_ref!(expected_out, 0, 32));
             assert_eq!(hasher.finalize(), *array_ref!(test_out, 0, 32));
@@ -495,12 +495,12 @@ fn test_reset() {
         crate::keyed_hash(key, &[42; CHUNK_LEN + 3]),
     );
 
-    let context = "BLAKE3 2020-02-12 10:20:58 reset test";
-    let mut kdf = crate::Hasher::new_derive_key(context);
+    let purpose = "BLAKE3 2020-02-12 10:20:58 reset test";
+    let mut kdf = crate::Hasher::new_derive_key(purpose);
     kdf.update(&[42; 3 * CHUNK_LEN + 7]);
     kdf.reset();
     kdf.update(&[42; CHUNK_LEN + 3]);
-    let expected = crate::derive_key(context, &[42; CHUNK_LEN + 3]);
+    let expected = crate::derive_key(purpose, &[42; CHUNK_LEN + 3]);
     assert_eq!(kdf.finalize(), expected);
 }