// A.Greensted
// http://www.labbookpages.co.uk
// Version 1.0

public class EngNotation
{
	private final static int PREFIX_OFFSET = 5;
	private final static String[] PREFIX_ARRAY = {"f", "p", "n", "µ", "m", "", "k", "M", "G", "T"};

	private final static char[] PREFIX_TEST_ARRAY = {'f', 'p', 'n', 'u', 'µ', 'm', 'k', 'K', 'M', 'G', 'T'};
	private final static int[] PREFIX_EXP_ARRAY = {-15, -12, -9, -6, -6, -3, 3, 3, 6, 9, 12};

	// Converts a double into a 'Engineering notated' string
	// val:	The value to be converted
	// dp:	The number of decimal places
	public static String convert(double val, int dp)
	{
		// If the value is zero, then simply return 0 with the correct number of dp
		if (val == 0) return String.format("%." + dp + "f", 0.0);

		// If the value is negative, make it positive so the log10 works
		double posVal = (val<0) ? -val : val;
		double log10 = Math.log10(posVal);

		// Determine how many orders of 3 magnitudes the value is
		int count = (int) Math.floor(log10/3);

		// Calculate the index of the prefix symbol
		int index = count + PREFIX_OFFSET;

		// Scale the value into the range 1<=val<1000
		val /= Math.pow(10, count * 3);

		if (index >= 0 && index < PREFIX_ARRAY.length)
		{
			// If a prefix exists use it to create the correct string
			return String.format("%." + dp + "f%s", val, PREFIX_ARRAY[index]);
		}
		else
		{
			// If no prefix exists just make a string of the form 000e000
			return String.format("%." + dp + "fe%d", val, count * 3);
		}
	}

	public static double parse(String str)
	{
		return parse(str.toCharArray());
	}

	public static double parse(char[] chars) throws NumberFormatException
	{
		int exponent = 0;
		double value = 0;

		boolean gotChar = false;		// Set to true once any non-whitespace, or minus character has been found
		boolean gotMinus = false;		// Set to true once a minus character has been found
		boolean gotDP = false;			//	Set to true once a decimal place character has been found
		boolean gotPrefix = false;		// Set to true once a prefix character has been found
		boolean gotDigit = false;		// Set to true once a digit character has been found

		// Search for start of string
		int start = 0;
		while (start < chars.length) {
			if (chars[start] != ' ' && chars[start] != '\t') break;
			start ++;
		}

		if (start == chars.length) throw new NumberFormatException("Empty string");

		// Search for end of string
		int end = chars.length - 1;
		while (end >= 0) {
			if (chars[end] != ' ' && chars[end] != '\t') break;
			end --;
		}

		// Iterate through characters
		CharLoop: for (int c=start ; c<=end ; c++)
		{
			// Check for a minus symbol
			if (chars[c] == '-')
			{
				if (gotChar) throw new NumberFormatException("Can only have minus symbol at the start");
				if (gotMinus) throw new NumberFormatException("Too many minus symbols");
				gotMinus = true;
				continue CharLoop;
			}

			gotChar = true;

			// Check for a numerical digit
			if (chars[c] >= '0' && chars[c] <= '9')
			{
				if (gotPrefix || gotDP) exponent --;
				if (gotPrefix && gotDP) throw new NumberFormatException("Cannot have digits after prefix when number includes decimal point");
				value *= 10;
				value += chars[c] - '0';
				gotDigit = true;
				continue CharLoop;
			}

			// Check for a decimal place
			if (chars[c] == '.')
			{
				if (gotDP) throw new NumberFormatException("Too many decimal points");
				if (gotPrefix) throw new NumberFormatException("Cannot have decimal point after prefix");
				gotDP = true;
				continue CharLoop;
			}

			// Check for a match with a prefix character
			for (int p=0 ; p<PREFIX_TEST_ARRAY.length ; p++)
			{
				if (PREFIX_TEST_ARRAY[p] == chars[c])
				{
					if (gotPrefix) throw new NumberFormatException("Too many prefixes");
					exponent += PREFIX_EXP_ARRAY[p];
					gotPrefix = true;
					continue CharLoop;
				}
			}

			// All other characters are invalid
			throw new NumberFormatException("Invalid character '" + chars[c] + "'");
		}

		// Check if any digits were found
		if (!gotDigit) throw new NumberFormatException("No digits");

		// Apply negation if required
		if (gotMinus) value *= -1;

		return value * Math.pow(10, exponent);
	}
}