reactive-mail-server/backend/src/main/java/com/mailserver/imap/ImapReader.java

package com.mailserver.imap;

import java.nio.charset.StandardCharsets;
import java.util.ArrayList;
import java.util.List;

/**
 * Reads one IMAP command: the decoder has already collected it whole, with its literals inline
 * ("{n}\r\n" + n bytes, or "{n+}\r\n" + n bytes).
 */
final class ImapReader {

    static final class SyntaxError extends RuntimeException {
        SyntaxError(String message) {
            super(message);
        }
    }

    private final byte[] b;
    private int pos;

    ImapReader(byte[] command) {
        this(command, true);
    }

    private ImapReader(byte[] command, boolean stripLineEnd) {
        int end = command.length;
        if (!stripLineEnd) {
            this.b = command;
            return;
        }
        if (end > 0 && command[end - 1] == '\n') {
            end--;
        }
        if (end > 0 && command[end - 1] == '\r') {
            end--;
        }
        this.b = java.util.Arrays.copyOf(command, end);
    }

    boolean atEnd() {
        return pos >= b.length;
    }

    int peek() {
        return pos < b.length ? b[pos] & 0xff : -1;
    }

    void sp() {
        if (peek() != ' ') {
            throw new SyntaxError("Expected a space");
        }
        while (peek() == ' ') {
            pos++;
        }
    }

    boolean optionalSp() {
        if (peek() == ' ') {
            sp();
            return true;
        }
        return false;
    }

    void expect(char c) {
        if (peek() != c) {
            throw new SyntaxError("Expected '" + c + "'");
        }
        pos++;
    }

    /** Characters up to a space, parenthesis, or the end (an atom; also tags, sequence sets, flags). */
    String atom() {
        int start = pos;
        while (pos < b.length) {
            int c = b[pos] & 0xff;
            if (c == ' ' || c == '(' || c == ')' || c == '{' || c == '"' || c < 0x20 || c == 0x7f) {
                break;
            }
            pos++;
        }
        if (pos == start) {
            throw new SyntaxError("Expected an atom");
        }
        return new String(b, start, pos - start, StandardCharsets.UTF_8);
    }

    /** Atom, quoted string or literal. */
    String astring() {
        int c = peek();
        if (c == '"' || c == '{') {
            return new String(stringBytes(), StandardCharsets.UTF_8);
        }
        return atom();
    }

    /** Quoted string or literal, raw bytes. */
    byte[] stringBytes() {
        int c = peek();
        if (c == '"') {
            pos++;
            java.io.ByteArrayOutputStream out = new java.io.ByteArrayOutputStream();
            while (true) {
                if (pos >= b.length) {
                    throw new SyntaxError("Unterminated quoted string");
                }
                byte x = b[pos++];
                if (x == '"') {
                    return out.toByteArray();
                }
                if (x == '\\' && pos < b.length) {
                    x = b[pos++];
                }
                out.write(x);
            }
        }
        if (c == '{') {
            int close = indexOf('}', pos);
            if (close < 0) {
                throw new SyntaxError("Bad literal");
            }
            String size = new String(b, pos + 1, close - pos - 1, StandardCharsets.US_ASCII);
            if (size.endsWith("+")) {
                size = size.substring(0, size.length() - 1);
            }
            int length;
            try {
                length = Integer.parseInt(size);
            } catch (NumberFormatException e) {
                throw new SyntaxError("Bad literal size");
            }
            pos = close + 1;
            if (peek() == '\r') {
                pos++;
            }
            expect('\n');
            if (pos + length > b.length) {
                throw new SyntaxError("Literal shorter than announced");
            }
            byte[] data = java.util.Arrays.copyOfRange(b, pos, pos + length);
            pos += length;
            return data;
        }
        throw new SyntaxError("Expected a string");
    }

    /** "(a b c)" → [a, b, c] (atoms or strings); an atom without parentheses → [atom]. */
    List<String> list() {
        List<String> items = new ArrayList<>();
        if (peek() != '(') {
            items.add(astring());
            return items;
        }
        pos++;
        while (peek() == ' ') {
            pos++;
        }
        while (peek() != ')') {
            if (atEnd()) {
                throw new SyntaxError("Unterminated list");
            }
            items.add(astring());
            while (peek() == ' ') {
                pos++;
            }
        }
        pos++;
        return items;
    }

    /** Everything left, as text (used for FETCH attributes and SEARCH keys parsed by their own readers). */
    String rest() {
        String text = new String(b, pos, b.length - pos, StandardCharsets.UTF_8);
        pos = b.length;
        return text;
    }

    /** A reader over the rest, as is (SEARCH may hold literals). */
    ImapReader restReader() {
        ImapReader reader = new ImapReader(java.util.Arrays.copyOfRange(b, pos, b.length), false);
        pos = b.length;
        return reader;
    }

    private int indexOf(char c, int from) {
        for (int i = from; i < b.length; i++) {
            if (b[i] == c) {
                return i;
            }
        }
        return -1;
    }
}