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package hirondelle.date4j;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 Convert a date-time from a string into a  {@link DateTime}.
 The primary use case for this class is converting date-times from a database ResultSet
 into a {@link DateTime}. It can also convert an ISO time, having a 'T' separating the date 
 from the time.
*/
final class DateTimeParser  {

  /** 
   Thrown when the given string cannot be converted into a DateTime, since it doesn't 
   have a format allowed by this class. 
   An unchecked exception.
  */
  static final class UnknownDateTimeFormat extends RuntimeException {
    private static final long serialVersionUID = -7179421566055773208L;
    UnknownDateTimeFormat(String aMessage){   super(aMessage);   }
    UnknownDateTimeFormat(String aMessage, Throwable aEx){   super(aMessage, aEx);   }
  }
  
  DateTime parse(String aDateTime) {
    if(aDateTime == null){
      throw new NullPointerException("DateTime string is null");
    }
    String dateTime = aDateTime.trim();
    Parts parts = splitIntoDateAndTime(dateTime);
    if (parts.hasTwoParts()) {
      parseDate(parts.datePart);
      parseTime(parts.timePart);
    }
    else if (parts.hasDateOnly()){
      parseDate(parts.datePart);
    }
    else if (parts.hasTimeOnly()){
      parseTime(parts.timePart);
    }
    DateTime result = new DateTime(fYear, fMonth, fDay, fHour, fMinute, fSecond, fNanosecond);
    return result;
  }
  
  // PRIVATE
  
  /** 
   Gross pattern for dates. 
   Detailed validation is done by DateTime.
   The Group index VARIES for y-m-d according to which option is selected
   Year: Group 1, 4, 6
   Month: Group 2, 5
   Day: Group 3 
  */
  private static final Pattern DATE = Pattern.compile("(\\d{1,4})-(\\d\\d)-(\\d\\d)|(\\d{1,4})-(\\d\\d)|(\\d{1,4})");
  
  /** 
   Gross pattern for times. 
   Detailed validation is done by DateTime.   
   The Group index VARIES for h-m-s-f according to which option is selected
   Hour: Group 1, 5, 8, 10
   Minute: Group 2, 6, 9
   Second: Group 3, 7
   Microsecond:  Group 4
  */
  private static final String CL = "\\:"; //colon is a special character
  private static final String TT = "(\\d\\d)"; //colon is a special character
  private static final String NUM_DIGITS_FOR_FRACTIONAL_SECONDS = "9";
  private static final Integer NUM_DIGITS = Integer.valueOf(NUM_DIGITS_FOR_FRACTIONAL_SECONDS);
  private static final Pattern TIME = Pattern.compile("" +
      TT+CL+TT+CL+TT+ "\\." + "(\\d{1," + NUM_DIGITS_FOR_FRACTIONAL_SECONDS + "})" + "|" + 
      TT+CL+TT+CL+TT+ "|" + 
      TT+CL+TT+ "|" +
      TT
  );
  
  private static final String COLON = ":";
  private static final int THIRD_POSITION = 2;
  
  private Integer fYear;
  private Integer fMonth;
  private Integer fDay;
  private Integer fHour;
  private Integer fMinute;
  private Integer fSecond;
  private Integer fNanosecond;
  
  private class Parts {
    String datePart;
    String timePart;
    boolean hasTwoParts(){
      return datePart != null && timePart != null;
    }
    boolean hasDateOnly(){
      return timePart == null;
    }
    boolean hasTimeOnly(){
      return datePart == null;
    }
  }
  
  /** Date and time can be separated with a single space, or with a 'T' character (case-sensitive). */
  private Parts splitIntoDateAndTime(String aDateTime){
    Parts result = new Parts();
    int dateTimeSeparator = getDateTimeSeparator(aDateTime);
    boolean hasDateTimeSeparator = 0 < dateTimeSeparator  && dateTimeSeparator < aDateTime.length();
    if (hasDateTimeSeparator){
      result.datePart = aDateTime.substring(0, dateTimeSeparator);
      result.timePart = aDateTime.substring(dateTimeSeparator+1);
    }
    else if(hasColonInThirdPlace(aDateTime)){
      result.timePart = aDateTime;
    }
    else {
      result.datePart = aDateTime;
    }
    return result;
  }
  
  /** Return the index of a space character, or of a 'T' character. If not found, return -1.*/
  int getDateTimeSeparator(String aDateTime){
    String SPACE = " ";
    int NOT_FOUND = -1;
    int result = NOT_FOUND;
    result = aDateTime.indexOf(SPACE);
    if(result == NOT_FOUND){
      result = aDateTime.indexOf("T");
    }
    return result;
  }
  
  private boolean hasColonInThirdPlace(String aDateTime){
    boolean result = false;
    if(aDateTime.length() >= THIRD_POSITION){
      result = COLON.equals(aDateTime.substring(THIRD_POSITION,THIRD_POSITION+1));
    }
    return result;
  }
  
  private void parseDate(String aDate) {
    Matcher matcher = DATE.matcher(aDate);
    if (matcher.matches()){
      String year = getGroup(matcher, 1, 4, 6);
      if(year !=null ){
        fYear = Integer.valueOf(year);
      }
      String month = getGroup(matcher, 2, 5);
      if(month !=null ){
        fMonth = Integer.valueOf(month);
      }
      String day = getGroup(matcher, 3);
      if(day !=null ){
        fDay = Integer.valueOf(day);
      }
    }
    else {
      throw new DateTimeParser.UnknownDateTimeFormat("Unexpected format for date:" + aDate);
    }
  }

  private String getGroup(Matcher aMatcher, int... aGroupIds){
    String result = null;
    for(int id: aGroupIds){
      result = aMatcher.group(id);
      if(result!=null) break;
    }
    return result;
  }

  private void parseTime(String aTime) {
    Matcher matcher = TIME.matcher(aTime);
    if (matcher.matches()){
      String hour = getGroup(matcher, 1, 5, 8, 10);
      if(hour !=null ){
        fHour = Integer.valueOf(hour);
      }
      String minute = getGroup(matcher, 2, 6, 9);
      if(minute !=null ){
        fMinute = Integer.valueOf(minute);
      }
      String second = getGroup(matcher, 3, 7);
      if(second !=null ){
        fSecond = Integer.valueOf(second);
      }
      String decimalSeconds = getGroup(matcher, 4);
      if(decimalSeconds !=null ){
        fNanosecond = Integer.valueOf(convertToNanoseconds(decimalSeconds));
      }
    }
    else {
      throw new DateTimeParser.UnknownDateTimeFormat("Unexpected format for time:" + aTime);
    }
  }
  
  /**
   Convert any number of decimals (1..9) into the form it would have taken if nanos had been used, 
   by adding any 0's to the right side.  
  */
  private String convertToNanoseconds(String aDecimalSeconds){
    StringBuilder result = new StringBuilder(aDecimalSeconds);
    while( result.length( ) < NUM_DIGITS ){
      result.append("0");
    }
    return result.toString();
  }
}