add try_read function to mirror try_unread

3 files changed, 31 insertions, 5 deletions

diff --git a/vespalib/src/tests/data/input_reader/input_reader_test.cpp b/vespalib/src/tests/data/input_reader/input_reader_test.cpp
index c74b16400df..535c188d01e 100644
--- a/vespalib/src/tests/data/input_reader/input_reader_test.cpp
+++ b/vespalib/src/tests/data/input_reader/input_reader_test.cpp
@@ -138,4 +138,21 @@ TEST("require that bytes can be unread when appropriate") {
     EXPECT_TRUE(!src.failed());
 }
 
+TEST("require that try read finds eof without failing the reader") {
+    const char *data = "12345";
+    MemoryInput memory_input(data);
+    ChunkedInput input(memory_input, 3);
+    InputReader src(input);
+    EXPECT_EQUAL(src.try_read(), '1');
+    EXPECT_EQUAL(src.try_read(), '2');
+    EXPECT_EQUAL(src.try_read(), '3');
+    EXPECT_EQUAL(src.try_read(), '4');
+    EXPECT_EQUAL(src.try_read(), '5');
+    EXPECT_TRUE(src.try_unread());
+    EXPECT_EQUAL(src.try_read(), '5');
+    EXPECT_EQUAL(src.try_read(), '\0');
+    EXPECT_TRUE(!src.try_unread());
+    EXPECT_TRUE(!src.failed());
+}
+
 TEST_MAIN() { TEST_RUN_ALL(); }
diff --git a/vespalib/src/vespa/vespalib/data/input_reader.h b/vespalib/src/vespa/vespalib/data/input_reader.h
index 29f6b88b506..f92ced7e508 100644
--- a/vespalib/src/vespa/vespalib/data/input_reader.h
+++ b/vespalib/src/vespa/vespalib/data/input_reader.h
@@ -74,6 +74,19 @@ public:
     }
 
     /**
+     * Try to read a single byte. This function will not fail the
+     * reader with buffer underflow if eof is reached.
+     *
+     * @return the next input byte, or 0 if eof is reached
+     **/
+    char try_read() {
+        if (__builtin_expect(obtain() > 0, true)) {
+            return _data.data[_pos++];
+        }
+        return 0;
+    }
+
+    /**
      * Try to unread a single byte. This will work for data that is
      * read, but not yet evicted. Note that after eof is found (the
      * obtain function returns 0), unreading will not be possible.
diff --git a/vespalib/src/vespa/vespalib/data/slime/json_format.cpp b/vespalib/src/vespa/vespalib/data/slime/json_format.cpp
index 6eb314141c1..18bf5289f0d 100644
--- a/vespalib/src/vespa/vespalib/data/slime/json_format.cpp
+++ b/vespalib/src/vespa/vespalib/data/slime/json_format.cpp
@@ -180,11 +180,7 @@ struct JsonDecoder {
     JsonDecoder(InputReader &reader) : in(reader), c(in.read()), key(), value() {}
 
     void next() {
-        if (in.obtain() > 0) {
-            c = in.read();
-        } else {
-            c = 0;
-        }
+        c = in.try_read();
     }
 
     bool skip(char x) {