Implement #scan_integer to efficiently parse Integer

Fix: #113

This allows to directly parse an Integer from a String without needing
to first allocate a sub string.

Notes:

The implementation is limited by design, it's meant as a first step,
only the most straightforward, based 10 integers are supported.

ext/strscan/strscan.c

@@ -115,6 +115,7 @@ static VALUE strscan_get_byte _((VALUE self));
 static VALUE strscan_getbyte _((VALUE self));
 static VALUE strscan_peek _((VALUE self, VALUE len));
 static VALUE strscan_peep _((VALUE self, VALUE len));
+static VALUE strscan_scan_integer _((VALUE self));
 static VALUE strscan_unscan _((VALUE self));
 static VALUE strscan_bol_p _((VALUE self));
 static VALUE strscan_eos_p _((VALUE self));
@@ -1266,6 +1267,48 @@ strscan_peep(VALUE self, VALUE vlen)
     return strscan_peek(self, vlen);
 }
 
+/*
+ * call-seq:
+ *   scan_integer
+ *
+ * Equivalent to #scan with a \-?\d+ pattern, and returns an Integer or nil.
+ */
+static VALUE
+strscan_scan_integer(VALUE self)
+{
+    struct strscanner *p;
+
+    GET_SCANNER(self, p);
+
+    char *ptr = CURPTR(p);
+
+    long len = 0;
+    if (ptr[len] == '-' || ptr[len] == '+') {
+        len++;
+    }
+
+    if (!isdigit(ptr[len])) {
+        return Qnil;
+    }
+
+    MATCHED(p);
+    p->prev = p->curr;
+
+    while(isdigit(ptr[len])) {
+        len++;
+    }
+
+    VALUE buffer_v;
+    char *buffer = ALLOCV_N(char, buffer_v, len + 1);
+
+    MEMCPY(buffer, CURPTR(p), char, len);
+    buffer[len] = '\0';
+    VALUE integer = rb_cstr2inum(buffer, 10);
+    RB_GC_GUARD(buffer_v);
+    p->curr += len;
+    return integer;
+}
+
 /*
  * :markup: markdown
  * :include: strscan/link_refs.txt
@@ -2204,6 +2247,8 @@ Init_strscan(void)
     rb_define_method(StringScanner, "peek_byte",   strscan_peek_byte,   0);
     rb_define_method(StringScanner, "peep",        strscan_peep,        1);
 
+    rb_define_method(StringScanner, "scan_integer", strscan_scan_integer, 0);
+
     rb_define_method(StringScanner, "unscan",      strscan_unscan,      0);
 
     rb_define_method(StringScanner, "beginning_of_line?", strscan_bol_p, 0);

test/strscan/test_stringscanner.rb

@@ -890,6 +890,42 @@ def test_named_captures
     assert_equal(9, scan.match?(/(?<f>foo)(?<r>bar)(?<z>baz)/))
     assert_equal({"f" => "foo", "r" => "bar", "z" => "baz"}, scan.named_captures)
   end
+
+  def test_scan_integer
+    s = create_string_scanner('abc')
+    assert_nil s.scan_integer
+    assert_equal 0, s.pos
+
+    s = create_string_scanner('123abc')
+    assert_equal 123, s.scan_integer
+    assert_equal 3, s.pos
+
+    s = create_string_scanner('-123abc')
+    assert_equal -123, s.scan_integer
+    assert_equal 4, s.pos
+
+    s = create_string_scanner('+123')
+    assert_equal 123, s.scan_integer
+    assert_equal 4, s.pos
+
+    s = create_string_scanner('-abc')
+    assert_nil s.scan_integer
+    assert_equal 0, s.pos
+
+    huge_integer = '1' * 2_000
+    s = create_string_scanner(huge_integer)
+    assert_equal huge_integer.to_i, s.scan_integer
+    assert_equal 2_000, s.pos
+  end
+
+  def test_scan_integer_unmatch
+    s = create_string_scanner('123abc')
+    assert_equal 123, s.scan_integer
+    assert_equal 3, s.pos
+
+    s.unscan
+    assert_equal 0, s.pos
+  end
 end
 
 class TestStringScanner < Test::Unit::TestCase