restructure source and refactor set_text_body and set_html_body

src/header/encoding.rs

@@ -0,0 +1,48 @@
+use std::fmt::Write;
+use email_encoding::headers::writer::EmailWriter;
+
+fn allowed_str(s: &str) -> bool {
+    s.bytes().all(allowed_char)
+}
+
+const fn allowed_char(c: u8) -> bool {
+    c >= 1 && c <= 9 || c == 11 || c == 12 || c >= 14 && c <= 127
+}
+
+fn encode(encode_buf: &mut String, writer: &mut EmailWriter) {
+    if encode_buf.is_empty() {
+        return;
+    }
+
+    let prefix = encode_buf.trim_end_matches(' ');
+    email_encoding::headers::rfc2047::encode(prefix, writer).expect("String is always writable");
+
+    // TODO: add a better API for doing this in email-encoding
+    let spaces = encode_buf.len() - prefix.len();
+    for _ in 0..spaces {
+        writer.space();
+    }
+
+    encode_buf.clear();
+}
+
+pub fn encode_header_value(key: &str, value: &str) -> String {
+    let mut encoded_value = String::new();
+    let line_len = key.len() + ": ".len();
+    let mut writer = EmailWriter::new(&mut encoded_value, line_len, 0, false);
+    let mut encode_buf = String::new();
+    for next_word in value.split_inclusive(' ') {
+        if allowed_str(next_word) {
+            // This word only contains allowed characters
+            // the next word is allowed, but we may have accumulated some words to encode
+            encode(&mut encode_buf, &mut writer);
+            writer.folding().write_str(next_word).expect("String is always writable");
+        } else {
+            // This word contains unallowed characters
+            encode_buf.push_str(next_word);
+        }
+    }
+    encode(&mut encode_buf, &mut writer);
+    drop(writer);
+    encoded_value
+}