Implement source linker and add URL to source code in generated docs.

Long overdue.

crates/aiken-project/src/docs/source_links.rs

@@ -0,0 +1,84 @@
+use crate::{
+    config::{Config, Platform},
+    CheckedModule,
+};
+use aiken_lang::{ast::Span, line_numbers::LineNumbers};
+use camino::{Utf8Component, Utf8Path};
+use std::path::Path;
+
+pub struct SourceLinker {
+    line_numbers: LineNumbers,
+    url_pattern: Option<(String, String)>,
+}
+
+impl SourceLinker {
+    pub fn new(root: &Path, config: &Config, module: &CheckedModule) -> Self {
+        let utf8_path = <&Utf8Path>::try_from(
+            module
+                .input_path
+                .as_path()
+                .strip_prefix(root)
+                .expect("root path isn't a prefix of project modules' paths!"),
+        )
+        .expect("module path contains non UTF-8 characters!");
+
+        let path_in_repo = to_url_path(utf8_path).unwrap_or_default();
+
+        let url_pattern = config
+            .repository
+            .as_ref()
+            .map(|repository| match repository.platform {
+                Platform::Github => (
+                    format!(
+                        "https://github.com/{}/{}/blob/{}/{}#L",
+                        repository.user, repository.project, config.version, path_in_repo
+                    ),
+                    "-L".into(),
+                ),
+                Platform::Gitlab => (
+                    format!(
+                        "https://gitlab.com/{}/{}/-/blob/{}/{}#L",
+                        repository.user, repository.project, config.version, path_in_repo
+                    ),
+                    "-".into(),
+                ),
+                Platform::Bitbucket => (
+                    format!(
+                        "https://bitbucket.com/{}/{}/src/{}/{}#lines-",
+                        repository.user, repository.project, config.version, path_in_repo
+                    ),
+                    ":".into(),
+                ),
+            });
+
+        SourceLinker {
+            line_numbers: LineNumbers::new(&module.code),
+            url_pattern,
+        }
+    }
+
+    pub fn url(&self, span: Span) -> String {
+        match &self.url_pattern {
+            Some((base, line_sep)) => {
+                let start_line = self.line_numbers.line_number(span.start).unwrap();
+                let end_line = self.line_numbers.line_number(span.end).unwrap();
+                format!("{base}{start_line}{line_sep}{end_line}")
+            }
+            None => "".into(),
+        }
+    }
+}
+
+fn to_url_path(path: &Utf8Path) -> Option<String> {
+    let mut buf = String::new();
+    for c in path.components() {
+        if let Utf8Component::Normal(s) = c {
+            buf.push_str(s);
+        }
+        buf.push('/');
+    }
+
+    let _ = buf.pop();
+
+    Some(buf)
+}