string: add `strip_margin`

vlib/builtin/string.v

@@ -1267,3 +1267,64 @@ pub fn (s string) repeat(count int) string {
 	ret[s.len * count] = 0
 	return string(ret)
 }
+
+// Allows multi-line strings to be formatted in a way that removes white-space
+// before a delimeter. by default `|` is used.
+// Note: the delimiter has to be a byte at this time. That means surrounding
+// the value in ``.
+// 
+// Example:
+// st := 'Hello there,
+//       |this is a string,
+//       |    Everything before the first | is removed'.strip_margin()
+// Returns:
+// Hello there,
+// this is a string,
+//     Everything before the first | is removed
+pub fn (s string) strip_margin(del ...byte) string {
+	mut sep := `|`
+	if del.len >= 1 {
+		// This is a workaround. We can't directly index a var_args array.
+		// Only care about the first one, ignore the rest if more
+		for d in del {
+			// The delimiter is not allowed to be white-space. Will use default
+			if !d.is_space() {
+				sep = d
+			} else {
+				eprintln("Warning: `strip_margin` cannot use white-space as a delimiter")
+				eprintln("    Defaulting to `|`")
+			}
+			break
+		}
+		if del.len == 1 {
+			eprintln("Warning: `strip_margin` only uses the first argument given")
+		}
+	}
+	// don't know how much space the resulting string will be, but the max it
+	// can be is this big
+	mut ret := malloc(s.len + 1)
+	mut count := 0
+	for i := 0; i < s.len; i++ {
+		if (s[i] in [`\n`, `\r`]) {
+			$if windows {
+				ret[count] = `\r`
+				ret[count+1] = `\n`
+				count += 2
+			} $else {
+				ret[count] = s[i]
+				count++
+			}
+			for s[i] != sep {
+				i++
+				if i >= s.len {
+					break
+				}
+			}
+		} else {
+			ret[count] = s[i]
+			count++
+		}
+	}
+	ret[count] = 0
+	return string(ret)
+}

vlib/builtin/string_test.v

@@ -695,3 +695,79 @@ fn test_split_into_lines() {
 		assert line == line_content
 	}
 }
+
+fn test_strip_margins() {
+	no_tabs := 'Hello there
+This is a string
+With multiple lines'
+	no_tabs_stripped := 'Hello there
+	                    |This is a string
+						|With multiple lines'.strip_margin()
+	assert no_tabs == no_tabs_stripped
+
+	text_before := 'There is text
+before the delimiter
+that should be removed as well'
+	text_before_stripped := 'There is text
+	f lasj  asldfj j lksjdf |before the delimiter
+	Which is removed hello  |that should be removed as well'.strip_margin()
+	assert text_before_stripped == text_before
+
+	tabs := '	Tab
+    spaces
+	another tab'
+	tabs_stripped := '	Tab
+	                 |    spaces
+					 |	another tab'.strip_margin()
+	assert tabs == tabs_stripped
+
+	alternate_delimiter := 'This has a different delim,
+but that is ok
+	because everything works'
+	alternate_delimiter_stripped := 'This has a different delim,
+		                            #but that is ok
+                                    #	because everything works'.strip_margin(`#`)
+	assert alternate_delimiter_stripped == alternate_delimiter
+
+	delim_after_first_instance := 'The delimiter used
+only matters the |||| First time it is seen
+not any | other | times'
+	delim_after_first_instance_stripped := 'The delimiter used
+	                                       |only matters the |||| First time it is seen
+	                                       |not any | other | times'.strip_margin()
+	assert delim_after_first_instance_stripped == delim_after_first_instance
+
+	uneven_delims := 'It doesn\'t matter if the delims are uneven,
+The text will still be delimited correctly.
+Maybe not everything needs 3 lines?
+Let us go for 4 then'
+	uneven_delims_stripped := 'It doesn\'t matter if the delims are uneven,
+           |The text will still be delimited correctly.
+                      |Maybe not everything needs 3 lines?
+		 	 	|Let us go for 4 then'.strip_margin()
+	assert uneven_delims_stripped == uneven_delims
+
+	multi_blank_lines := 'Multiple blank lines will be removed.
+	I actually consider this a feature.'
+	multi_blank_lines_stripped := 'Multiple blank lines will be removed.
+
+
+
+		|	I actually consider this a feature.'.strip_margin()
+	assert multi_blank_lines == multi_blank_lines_stripped
+
+	end_with_newline := 'This line will end with a newline
+Something cool or something.
+'
+	end_with_newline_stripped := 'This line will end with a newline
+	                             |Something cool or something.
+                                 
+					'.strip_margin()
+	assert end_with_newline_stripped == end_with_newline
+
+	space_delimiter := 'Using a white-space char will
+revert back to default behavior.'
+	space_delimiter_stripped := 'Using a white-space char will
+		|revert back to default behavior.'.strip_margin(`\n`)
+	assert space_delimiter == space_delimiter_stripped
+}