Improve JSON import performance by another 10%

Profiling showed that string parsing was a bottleneck. We rarely need
the full power of JSON strings, though, so we can optimize for the
common case of plain strings without escape codes. Keeping the slower
string parser as fallback, of course.

src/json_import.zig

@@ -82,11 +82,12 @@ const Parser = struct {
         };
     }
 
-    // Read a string (after the ") into buf.
+    fn stringContentSlow(p: *Parser, buf: []u8, head: u8, off: usize) []u8 {
-    // Any characters beyond the size of the buffer are consumed but otherwise discarded.
+        @setCold(true);
-    fn stringContent(p: *Parser, buf: []u8) []u8 {
+        var b = head;
-        var n: usize = 0;
+        var n = off;
-        while (true) switch (p.nextByte()) {
+        while (true) {
+            switch (b) {
                 '"' => break,
                 '\\' => switch (p.nextByte()) {
                     '"' => if (n < buf.len) { buf[n] = '"'; n += 1; },
@@ -104,12 +105,29 @@ const Parser = struct {
                     },
                     else => p.die("invalid escape sequence"),
                 },
-            0x20, 0x21, 0x23...0x5b, 0x5d...0xff => |b| if (n < buf.len) { buf[n] = b; n += 1; },
+                0x20, 0x21, 0x23...0x5b, 0x5d...0xff => if (n < buf.len) { buf[n] = b; n += 1; },
                 else => p.die("invalid character in string"),
-        };
+            }
+            b = p.nextByte();
+        }
         return buf[0..n];
     }
 
+    // Read a string (after the ") into buf.
+    // Any characters beyond the size of the buffer are consumed but otherwise discarded.
+    fn stringContent(p: *Parser, buf: []u8) []u8 {
+        // The common case (for ncdu dumps): string fits in the given buffer and does not contain any escapes.
+        var n: usize = 0;
+        var b = p.nextByte();
+        while (n < buf.len and b >= 0x20 and b != '"' and b != '\\') {
+            buf[n] = b;
+            n += 1;
+            b = p.nextByte();
+        }
+        if (b == '"') return buf[0..n];
+        return p.stringContentSlow(buf, b, n);
+    }
+
     fn string(p: *Parser, buf: []u8) []u8 {
         if (p.nextChr() != '"') p.die("expected string");
         return p.stringContent(buf);