/* Copyright 2002-2026 CS GROUP
    * Licensed to CS GROUP (CS) under one or more
    * contributor license agreements.  See the NOTICE file distributed with
    * this work for additional information regarding copyright ownership.
    * CS licenses this file to You under the Apache License, Version 2.0
    * (the "License"); you may not use this file except in compliance with
    * the License.  You may obtain a copy of the License at
    *
    *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.orekit.utils.units;
  
  import org.hipparchus.fraction.Fraction;
  import org.orekit.errors.OrekitException;
  import org.orekit.errors.OrekitMessages;
  
  /** Lexer for units.
   * @author Luc Maisonobe
   * @since 11.0
   */
  class Lexer {
  
      /** Unit specification to tokenize. */
      private final CharSequence unitSpecification;
  
      /** End index. */
      private final int end;
  
      /** Start index for next token. */
      private int start;
  
      /** Next to last token emitted. */
      private Token nextToLast;
  
      /** Last token emitted. */
      private Token last;
  
      /** Upcoming token (which was pushed back). */
      private Token upcoming;
  
      /** Build a lexer for a unit specification.
       * @param unitSpecification unit specification to tokenize
       */
      Lexer(final CharSequence unitSpecification) {
          this.unitSpecification = unitSpecification;
          this.end               = unitSpecification.length();
          this.start             = 0;
          this.last              = null;
      }
  
      /** Get the complete unit specification.
       * @return complete unit specification
       */
      public String getUnitSpecification() {
          return unitSpecification.toString();
      }
  
      /** Push back last returned token.
       * <p>
       * This can be called only once
       * </p>
       */
      public void pushBack() {
          upcoming = last;
          last     = nextToLast;
      }
  
      /** Get next token.
       * @return next token, or null if there are no more tokens
       */
      public Token next() {
  
          if (upcoming != null) {
              nextToLast = last;
              last       = upcoming;
              upcoming   = null;
              return last;
          }
  
          // skip whitespace
          while (start < end && Character.isWhitespace(unitSpecification.charAt(start))) {
              ++start;
          }
  
          if (start >= end) {
              // no more characters to analyze
              nextToLast = last;
              last       = null;
              return null;
          }
  
          // look for prefixed units
          int current = start;
         while (current < end &&
                (Character.isLowerCase(unitSpecification.charAt(current)) ||
                 Character.isUpperCase(unitSpecification.charAt(current)) ||
                 unitSpecification.charAt(current) == '°'  ||
                 unitSpecification.charAt(current) == '◦'  ||
                 unitSpecification.charAt(current) == '′'  ||
                 unitSpecification.charAt(current) == '\'' ||
                 unitSpecification.charAt(current) == '″'  ||
                 unitSpecification.charAt(current) == '"'  ||
                 unitSpecification.charAt(current) == '%'  ||
                 unitSpecification.charAt(current) == '#')) {
             ++current;
         }
         if (current > start) {
             return emit(current, TokenType.IDENTIFIER, 0, 1);
         }
 
         // look for power
         if (start < end - 1 &&
             unitSpecification.charAt(start)     == '*' &&
             unitSpecification.charAt(start + 1) == '*') {
             // power indicator as **
             return emit(start + 2, TokenType.POWER, 0, 1);
         } else if (unitSpecification.charAt(start) == '^') {
             // power indicator as ^
             return emit(start + 1, TokenType.POWER, 0, 1);
         } else if (convertSuperscript(start) != ' ' &&
                    last != null &&
                    last.getType() != TokenType.POWER) {
             // virtual power indicator as we switch to superscript characters
             return emit(start, TokenType.POWER, 0, 1);
         }
 
         // look for one character tokens
         if (unitSpecification.charAt(start) == '*') {
             return emit(start + 1, TokenType.MULTIPLICATION, 0, 1);
         } else if (unitSpecification.charAt(start) == '×') {
             return emit(start + 1, TokenType.MULTIPLICATION, 0, 1);
         } else if (unitSpecification.charAt(start) == '.') {
             return emit(start + 1, TokenType.MULTIPLICATION, 0, 1);
         } else if (unitSpecification.charAt(start) == '·') {
             return emit(start + 1, TokenType.MULTIPLICATION, 0, 1);
         } else if (unitSpecification.charAt(start) == '/') {
             return emit(start + 1, TokenType.DIVISION, 0, 1);
         } else if (unitSpecification.charAt(start) == '⁄') {
             return emit(start + 1, TokenType.DIVISION, 0, 1);
         } else if (unitSpecification.charAt(start) == '(') {
             return emit(start + 1, TokenType.OPEN, 0, 1);
         } else if (unitSpecification.charAt(start) == ')') {
             return emit(start + 1, TokenType.CLOSE, 0, 1);
         } else if (unitSpecification.charAt(start) == '√') {
             return emit(start + 1, TokenType.SQUARE_ROOT, 0, 1);
         }
 
         // look for special case "0.5" (used by CCSDS for square roots)
         if (start < end - 2 &&
              unitSpecification.charAt(start)     == '0' &&
              unitSpecification.charAt(start + 1) == '.' &&
              unitSpecification.charAt(start + 2) == '5') {
             // ½ written as decimal number
             return emit(start + 3, TokenType.FRACTION, 1, 2);
         }
 
         // look for special case "1.5" (used by CCSDS for power 3/2)
         if (start < end - 2 &&
              unitSpecification.charAt(start)     == '1' &&
              unitSpecification.charAt(start + 1) == '.' &&
              unitSpecification.charAt(start + 2) == '5') {
             // 3/2 written as decimal number
             return emit(start + 3, TokenType.FRACTION, 3, 2);
         }
 
         // look for unicode fractions
         if (unitSpecification.charAt(start) == '¼') {
             return emit(start + 1, TokenType.FRACTION, 1, 4);
         } else if (unitSpecification.charAt(start) == '½') {
             return emit(start + 1, TokenType.FRACTION, 1, 2);
         } else if (unitSpecification.charAt(start) == '¾') {
             return emit(start + 1, TokenType.FRACTION, 3, 4);
         } else if (unitSpecification.charAt(start) == '⅐') {
             return emit(start + 1, TokenType.FRACTION, 1, 7);
         } else if (unitSpecification.charAt(start) == '⅑') {
             return emit(start + 1, TokenType.FRACTION, 1, 9);
         } else if (unitSpecification.charAt(start) == '⅒') {
             return emit(start + 1, TokenType.FRACTION, 1, 10);
         } else if (unitSpecification.charAt(start) == '⅓') {
             return emit(start + 1, TokenType.FRACTION, 1, 3);
         } else if (unitSpecification.charAt(start) == '⅔') {
             return emit(start + 1, TokenType.FRACTION, 2, 3);
         } else if (unitSpecification.charAt(start) == '⅕') {
             return emit(start + 1, TokenType.FRACTION, 1, 5);
         } else if (unitSpecification.charAt(start) == '⅖') {
             return emit(start + 1, TokenType.FRACTION, 2, 5);
         } else if (unitSpecification.charAt(start) == '⅗') {
             return emit(start + 1, TokenType.FRACTION, 3, 5);
         } else if (unitSpecification.charAt(start) == '⅘') {
             return emit(start + 1, TokenType.FRACTION, 4, 5);
         } else if (unitSpecification.charAt(start) == '⅙') {
             return emit(start + 1, TokenType.FRACTION, 1, 6);
         } else if (unitSpecification.charAt(start) == '⅚') {
             return emit(start + 1, TokenType.FRACTION, 5, 6);
         } else if (unitSpecification.charAt(start) == '⅛') {
             return emit(start + 1, TokenType.FRACTION, 1, 8);
         } else if (unitSpecification.charAt(start) == '⅜') {
             return emit(start + 1, TokenType.FRACTION, 3, 8);
         } else if (unitSpecification.charAt(start) == '⅝') {
             return emit(start + 1, TokenType.FRACTION, 5, 8);
         } else if (unitSpecification.charAt(start) == '⅞') {
             return emit(start + 1, TokenType.FRACTION, 7, 8);
         }
 
         // it must be an integer, either as regular character or as superscript
         final Converter converter = (convertSuperscript(start) == ' ') ?
                                     this::noConvert :
                                     this::convertSuperscript;
 
         // manage sign, taking care of counting characters properly
         final int sign;
         final int numberStart;
         if (converter.convert(start) == '+') {
             sign        = +1;
             numberStart = start + 1;
         } else if (converter.convert(start) == '-') {
             sign        = -1;
             numberStart = start + 1;
         } else {
             sign        = 1;
             numberStart = start;
         }
         current = numberStart;
 
         int value = 0;
         while (current < end) {
             final int c = converter.convert(current);
             if (c >= '0' && c <= '9') {
                 value = value * 10 + (c - '0');
                 ++current;
             } else {
                 break;
             }
         }
         if (current > numberStart) {
             // there were some digits
             return emit(current, TokenType.INTEGER, sign * value, 1);
         }
 
         throw generateException();
 
     }
 
     /** Generate an exception.
      * @return generated exception
      */
     public OrekitException generateException() {
         return new OrekitException(OrekitMessages.UNKNOWN_UNIT, unitSpecification);
     }
 
     /** Emit one token.
      * @param after index after token
      * @param type token type
      * @param numerator value of the token numerator
      * @param denominator value of the token denominator
      * @return new token
      */
     private Token emit(final int after, final TokenType type, final int numerator, final int denominator) {
         final CharSequence subString = unitSpecification.subSequence(start, after);
         start      = after;
         nextToLast = last;
         last       = new Token(subString, type, numerator,
                                denominator == 1 ? null : new Fraction(numerator, denominator));
         return last;
     }
 
     /** Convert a superscript character to regular digit or sign character.
      * @param index character index
      * @return regular digit or sign character, or ' ' if character is not a superscript
      */
     private char convertSuperscript(final int index) {
         // we can't do fancy stuff with code points
         // superscripts for 1, 2 and 3 are not in the same range as others
         return switch (unitSpecification.charAt(index)) {
             case '⁰'  -> '0';
             case '¹'  -> '1';
             case '²'  -> '2';
             case '³'  -> '3';
             case '⁴'  -> '4';
             case '⁵'  -> '5';
             case '⁶'  -> '6';
             case '⁷'  -> '7';
             case '⁸'  -> '8';
             case '⁹'  -> '9';
             case '⁺'  -> '+';
             case '⁻'  -> '-';
             default  -> ' ';
         };
 
     }
 
     /** No-op converter.
      * @param index character index
      * @return character at index
      */
     private char noConvert(final int index) {
         return unitSpecification.charAt(index);
     }
 
     /** Character converter. */
     private interface Converter {
         /** Convert a character.
          * @param index character index
          * @return converted character
          */
         char convert(int index);
     }
 
 }