Add gettext command to generate translated output

This command is the second part of a Gettext-based translation (i18n)
workflow. It takes an `xx.po` file with translations and uses this to
translate the chapters of the book. Paragraphs without a translation
are kept in the original language.

Part of the solution for #5.

src/cmd/gettext.rs

@@ -0,0 +1,82 @@
+use crate::cmd::xgettext::extract_paragraphs;
+use crate::get_book_dir;
+use crate::utils;
+use clap::{arg, App, Arg, ArgMatches};
+use mdbook::MDBook;
+use polib::po_file::parse;
+use std::path::Path;
+
+// Create clap subcommand arguments
+pub fn make_subcommand<'help>() -> App<'help> {
+    App::new("gettext")
+        .about("Output translated book")
+        .arg(
+            Arg::new("dest-dir")
+                .short('d')
+                .long("dest-dir")
+                .value_name("dest-dir")
+                .help(
+                    "Output directory for the translated book{n}\
+                     Relative paths are interpreted relative to the book's root directory{n}\
+                     If omitted, mdBook defaults to `./src/xx` where `xx` is the language of the PO file."
+                ),
+        )
+        .arg(arg!(<po> "PO file to generate translation for"))
+        .arg(arg!([dir]
+            "Root directory for the book{n}\
+            (Defaults to the Current Directory when omitted)"
+        ))
+}
+
+// Gettext command implementation
+pub fn execute(args: &ArgMatches) -> mdbook::errors::Result<()> {
+    let book_dir = get_book_dir(args);
+    let book = MDBook::load(&book_dir)?;
+
+    let po_file = Path::new(args.value_of("po").unwrap());
+    let lang = po_file.file_stem().unwrap();
+    let catalog = parse(po_file).unwrap();
+    let dest_dir = book.root.join(
+        args.value_of("dest-dir")
+            .map(|path| path.into())
+            .unwrap_or(Path::new("src").join(lang)),
+    );
+
+    for item in book.iter() {
+        match item {
+            mdbook::BookItem::Chapter(chapter) if !chapter.is_draft_chapter() => {
+                let mut output = String::with_capacity(chapter.content.len());
+                let mut current_lineno = 1;
+
+                for (lineno, paragraph) in extract_paragraphs(&chapter.content) {
+                    // Fill in blank lines between paragraphs. This is
+                    // important for code blocks where blank lines can
+                    // be significant.
+                    while current_lineno < lineno {
+                        output.push('\n');
+                        current_lineno += 1;
+                    }
+                    current_lineno += paragraph.lines().count();
+
+                    let translated = catalog
+                        .find_message(paragraph)
+                        .and_then(|msg| msg.get_msgstr().ok())
+                        .filter(|msgstr| !msgstr.is_empty())
+                        .map(|msgstr| msgstr.as_str())
+                        .unwrap_or(paragraph);
+                    output.push_str(translated);
+                    output.push('\n');
+                }
+
+                utils::fs::write_file(
+                    &dest_dir,
+                    chapter.source_path.as_ref().unwrap(),
+                    output.as_bytes(),
+                )?;
+            }
+            _ => {}
+        }
+    }
+
+    Ok(())
+}

src/cmd/mod.rs

@@ -9,3 +9,4 @@ pub mod test;
 #[cfg(feature = "watch")]
 pub mod watch;
 pub mod xgettext;
+pub mod gettext;

src/main.rs

@@ -35,6 +35,7 @@ fn main() {
         Some(("serve", sub_matches)) => cmd::serve::execute(sub_matches),
         Some(("test", sub_matches)) => cmd::test::execute(sub_matches),
         Some(("xgettext", sub_matches)) => cmd::xgettext::execute(sub_matches),
+        Some(("gettext", sub_matches)) => cmd::gettext::execute(sub_matches),
         Some(("completions", sub_matches)) => (|| {
             let shell: Shell = sub_matches
                 .value_of("shell")
@@ -78,6 +79,7 @@ fn create_clap_app() -> App<'static> {
         .subcommand(cmd::test::make_subcommand())
         .subcommand(cmd::clean::make_subcommand())
         .subcommand(cmd::xgettext::make_subcommand())
+        .subcommand(cmd::gettext::make_subcommand())
         .subcommand(
             App::new("completions")
                 .about("Generate shell completions for your shell to stdout")