+package main
+
+import (
+ "strings"
+ "unicode"
+ "unicode/utf8"
+)
+
+// markdownPunctuation holds the ASCII characters that a backslash makes plain.
+const markdownPunctuation = "!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~"
+
+// MarkdownSpan is an inline mark of a line: the mark kind, the content inside
+// the marks, the index after the closing mark, and the URL of a link.
+type MarkdownSpan struct {
+ kind string
+ content string
+ end int
+ url string
+}
+
+// markdownCodeFence returns the language of a line that opens a fenced code
+// block, and the fence. The fence is "" when line does not open a block.
+func markdownCodeFence(line string) (string, string) {
+ trimmed := strings.TrimLeft(line, " ")
+ if len(line)-len(trimmed) > 3 || !strings.HasPrefix(trimmed, "```") {
+ return "", ""
+ }
+
+ run := len(trimmed) - len(strings.TrimLeft(trimmed, "`"))
+ info := strings.TrimSpace(trimmed[run:])
+ if strings.Contains(info, "`") {
+ return "", ""
+ }
+
+ language, _, _ := strings.Cut(info, " ")
+
+ return language, trimmed[:run]
+}
+
+// markdownClosingFence returns the index of the line after index that closes
+// the code block of fence, or -1 when fence is "" or no line closes it.
+func markdownClosingFence(lines []string, index int, fence string) int {
+ if fence == "" {
+ return -1
+ }
+
+ for end := index + 1; end < len(lines); end++ {
+ if markdownClosesFence(lines[end], fence) {
+ return end
+ }
+ }
+
+ return -1
+}
+
+// markdownClosesFence tells if line closes the code block of fence.
+func markdownClosesFence(line, fence string) bool {
+ trimmed := strings.TrimSpace(line)
+ return strings.HasPrefix(trimmed, fence) && strings.Trim(trimmed, "`") == ""
+}
+
+// markdownHeadingText returns the text of an ATX heading line without its
+// marks, or "" when line is not a heading or has no text.
+func markdownHeadingText(line string) string {
+ trimmed := strings.TrimLeft(line, " ")
+ if len(line)-len(trimmed) > 3 {
+ return ""
+ }
+
+ level := len(trimmed) - len(strings.TrimLeft(trimmed, "#"))
+ if level == 0 || level > 6 {
+ return ""
+ }
+
+ rest := trimmed[level:]
+ if rest != "" && rest[0] != ' ' && rest[0] != '\t' {
+ return ""
+ }
+
+ rest = strings.TrimSpace(rest)
+ closed := strings.TrimRight(rest, "#")
+ if closed == "" || strings.HasSuffix(closed, " ") {
+ rest = strings.TrimSpace(closed)
+ }
+
+ return rest
+}
+
+// markdownMarkSpan returns the inline mark that starts at index of line and
+// closes in line. It returns false when no mark starts there or the mark does
+// not close.
+func markdownMarkSpan(line string, index int) (MarkdownSpan, bool) {
+ switch line[index] {
+ case '`':
+ return markdownCodeSpan(line, index)
+ case '[':
+ return markdownLinkSpan(line, index)
+ case '~':
+ return markdownDelimited(line, index, "~~", "strikethrough")
+ case '*', '_':
+ double := line[index : index+1]
+ double += double
+ span, found := markdownDelimited(line, index, double, "bold")
+ if found {
+ return span, true
+ }
+
+ return markdownDelimited(line, index, line[index:index+1], "italic")
+ default:
+ return MarkdownSpan{}, false
+ }
+}
+
+// markdownCodeSpan returns the inline code that starts at index: a run of
+// backticks, the code, and a run of the same length. One space on each side of
+// the code is left out, as in Markdown.
+func markdownCodeSpan(line string, index int) (MarkdownSpan, bool) {
+ run := markdownBackticks(line[index:])
+ for search := index + run; search < len(line); {
+ next := strings.IndexByte(line[search:], '`')
+ if next < 0 {
+ break
+ }
+
+ next += search
+ length := markdownBackticks(line[next:])
+ if length != run {
+ search = next + length
+ continue
+ }
+
+ content := line[index+run : next]
+ if strings.TrimSpace(content) == "" {
+ break
+ }
+
+ if len(content) > 1 && content[0] == ' ' && content[len(content)-1] == ' ' {
+ content = content[1 : len(content)-1]
+ }
+
+ return MarkdownSpan{
+ kind: "code",
+ content: content,
+ end: next + run,
+ }, true
+ }
+
+ return MarkdownSpan{}, false
+}
+
+// markdownLinkSpan returns the link [text](url) that starts at index. The text
+// can hold brackets in pairs, and the URL holds no space.
+func markdownLinkSpan(line string, index int) (MarkdownSpan, bool) {
+ depth := 0
+ for closing := index; closing < len(line); closing++ {
+ switch line[closing] {
+ case '\\':
+ closing++
+ case '[':
+ depth++
+ case ']':
+ depth--
+ }
+
+ if depth > 0 {
+ continue
+ }
+
+ rest := line[closing+1:]
+ if closing == index+1 || !strings.HasPrefix(rest, "(") {
+ return MarkdownSpan{}, false
+ }
+
+ url, _, found := strings.Cut(rest[1:], ")")
+ if !found || url == "" || strings.ContainsAny(url, " \t") {
+ return MarkdownSpan{}, false
+ }
+
+ return MarkdownSpan{
+ kind: "text_link",
+ content: line[index+1 : closing],
+ end: closing + 2 + len(url) + 1,
+ url: url,
+ }, true
+ }
+
+ return MarkdownSpan{}, false
+}
+
+// markdownDelimited returns the span of kind between the mark at index and the
+// same mark later in line. An opening mark is followed by a character that is
+// not a space, and a closing mark follows one. A single mark is not a part of a
+// double one. A mark of underscores does not open or close inside a word, so
+// snake_case stays as text. A code span inside the content is skipped whole.
+func markdownDelimited(line string, index int, mark, kind string) (MarkdownSpan, bool) {
+ char := mark[0]
+ after := index + len(mark)
+ if !strings.HasPrefix(line[index:], mark) || after >= len(line) || markdownIsSpace(line[after]) {
+ return MarkdownSpan{}, false
+ }
+
+ if len(mark) == 1 && (line[after] == char || index > 0 && line[index-1] == char) {
+ return MarkdownSpan{}, false
+ }
+
+ if char == '_' && index > 0 && markdownWordCharBefore(line[:index]) {
+ return MarkdownSpan{}, false
+ }
+
+ for closing := after + 1; closing < len(line); closing++ {
+ switch {
+ case line[closing] == '\\':
+ closing++
+ continue
+ case line[closing] == '`':
+ span, found := markdownCodeSpan(line, closing)
+ if found {
+ closing = span.end - 1
+ }
+
+ continue
+ case !strings.HasPrefix(line[closing:], mark) || markdownIsSpace(line[closing-1]):
+ continue
+ }
+
+ end := closing + len(mark)
+ if len(mark) == 1 && (line[closing-1] == char || end < len(line) && line[end] == char) {
+ continue
+ }
+
+ if char == '_' && markdownWordChar(line[end:]) {
+ continue
+ }
+
+ return MarkdownSpan{
+ kind: kind,
+ content: line[after:closing],
+ end: end,
+ }, true
+ }
+
+ return MarkdownSpan{}, false
+}
+
+// markdownBackticks returns the length of the run of backticks at the start of
+// text.
+func markdownBackticks(text string) int {
+ return len(text) - len(strings.TrimLeft(text, "`"))
+}
+
+// markdownIsSpace tells if char is a space, a tab, or a line break.
+func markdownIsSpace(char byte) bool {
+ return char == ' ' || char == '\t' || char == '\n' || char == '\r'
+}
+
+// markdownWordCharBefore tells if the character that ends text is a letter or a
+// digit.
+func markdownWordCharBefore(text string) bool {
+ char, _ := utf8.DecodeLastRuneInString(text)
+ return text != "" && (unicode.IsLetter(char) || unicode.IsDigit(char))
+}
+
+// markdownWordChar tells if the character that starts text is a letter or a
+// digit. An underscore next to one is a part of a word, such as in snake_case.
+func markdownWordChar(text string) bool {
+ char, _ := utf8.DecodeRuneInString(text)
+ return text != "" && (unicode.IsLetter(char) || unicode.IsDigit(char))
+}
projects / onechat / commitdiff