Simplify history_output_buffer_t

This class is used to accumulate data to be written to the history
file. It has some dubious optimizations around trying to track an
offset separately from the size of the buffer. After some investigation
these aren't helping, vector behaves fine on its own. So just make
this a simple wrapper around vector.

src/history.cpp

@@ -21,6 +21,7 @@
 #include <cwchar>
 #include <iterator>
 #include <map>
+#include <numeric>
 
 #include "common.h"
 #include "env.h"
@@ -59,52 +60,44 @@ namespace {
 /// implement your own streambuf? Total insanity.
 static size_t safe_strlen(const char *s) { return s ? strlen(s) : 0; }
 class history_output_buffer_t {
-    // A null-terminated C string.
     std::vector<char> buffer;
-    // Offset is the offset of the null terminator.
-    size_t offset;
 
    public:
     /// Add a bit more to HISTORY_OUTPUT_BUFFER_SIZE because we flush once we've exceeded that size.
-    history_output_buffer_t() : buffer(HISTORY_OUTPUT_BUFFER_SIZE + 128, '\0'), offset(0) {}
+    explicit history_output_buffer_t(size_t reserve_amt = HISTORY_OUTPUT_BUFFER_SIZE + 128) {
+        buffer.reserve(reserve_amt);
+    }
 
     /// Append one or more strings.
     void append(const char *s1, const char *s2 = NULL, const char *s3 = NULL) {
-        const char *ptrs[4] = {s1, s2, s3, NULL};
-        const size_t lengths[4] = {safe_strlen(s1), safe_strlen(s2), safe_strlen(s3), 0};
+        constexpr size_t ptr_count = 3;
+        const char *ptrs[ptr_count] = {s1, s2, s3};
+        size_t lengths[ptr_count] = {safe_strlen(s1), safe_strlen(s2), safe_strlen(s3)};
 
         // Determine the additional size we'll need.
-        size_t additional_length = 0;
-        for (size_t i = 0; i < sizeof lengths / sizeof *lengths; i++) {
-            additional_length += lengths[i];
-        }
+        size_t additional_length = std::accumulate(std::begin(lengths), std::end(lengths), 0);
+        buffer.reserve(buffer.size() + additional_length);
 
-        // Allocate that much, plus a null terminator.
-        size_t required_size = offset + additional_length + 1;
-        if (required_size > buffer.size()) {
-            buffer.resize(required_size, '\0');
+        // Append
+        for (size_t i=0; i < ptr_count; i++) {
+            if (lengths[i] > 0) {
+                buffer.insert(buffer.end(), ptrs[i], ptrs[i] + lengths[i]);
+            }
         }
-
-        // Copy.
-        for (size_t i = 0; ptrs[i] != NULL; i++) {
-            memmove(&buffer.at(offset), ptrs[i], lengths[i]);
-            offset += lengths[i];
-        }
-
-        // Null terminator was appended by virtue of the resize() above (or in a previous
-        // invocation).
-        assert(buffer.at(buffer.size() - 1) == '\0');
     }
 
     /// Output to a given fd, resetting our buffer. Returns true on success, false on error.
     bool flush_to_fd(int fd) {
-        bool result = write_loop(fd, &buffer.at(0), offset) >= 0;
-        offset = 0;
+        if (buffer.empty()) {
+            return true;
+        }
+        bool result = write_loop(fd, &buffer.at(0), buffer.size()) >= 0;
+        buffer.clear();
         return result;
     }
 
     /// Return how much data we've accumulated.
-    size_t output_size() const { return offset; }
+    size_t output_size() const { return buffer.size(); }
 };
 
 class time_profiler_t {