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With inspiration from other libraries

There is a newer version: 2.12.15

/*
 * Copyright (c) 2012-2017 The ANTLR Project. All rights reserved.
 * Use of this file is governed by the BSD 3-clause license that
 * can be found in the LICENSE.txt file in the project root.
 */

package org.antlr.v4.gui;

/** Font metrics.  The only way to generate accurate images
 *  in any format that contain text is to know the font metrics.
 *  Specifically, we need to know the width of every character and the
 *  maximum height (since we want all characters to fit within same line height).
 *  I used ttf2tfm to dump the font metrics from Mac TrueType fonts and
 *  then converted that to a Java class for use in a PostScript generator
 *  for trees. Commands:
 *
 *  *	 $ ttf2tfm /Library/Fonts/Arial\ Black.ttf > metrics
 * 
 *
 * 	Then run metrics into python code after stripping header/footer:
 *
 *  *	 #
 *	 # Process lines from ttf2tfm that look like this:
 *	 # Glyph  Code   Glyph Name                Width  llx    lly      urx    ury
 *	 # ------------------------------------------------------------------------
 *	 #     3  00020  space                       333  0,     0 --     0,     0
 *	 #
 *	 lines = open("metrics").read().split('\n')
 *	 print "public class FontName {"
 *	 print "    {"
 *	 maxh = 0;
 *	 for line in lines[4:]: # skip header 0..3
 *			 all = line.split(' ')
 *			 words = [x for x in all if len(x)>0]
 *			 ascii = int(words[1], 16)
 *			 height = int(words[8])
 *			 if height>maxh: maxh = height
 *			 if ascii>=128: break
 *			 print "        widths[%d] = %s; // %s" % (ascii, words[3], words[2])
 *
 *	 print "        maxCharHeight = "+str(maxh)+";"
 *	 print "    }"
 *	 print "}"
 * 
 *
 *	Units are 1000th of an 'em'.
 */
public abstract class BasicFontMetrics {
	public static final int MAX_CHAR = '\u00FF';
	protected int maxCharHeight;
	protected int[] widths = new int[MAX_CHAR+1];

	public double getWidth(String s, int fontSize) {
		double w = 0;
		for (char c : s.toCharArray()) {
			w += getWidth(c, fontSize);
		}
		return w;
	}

	public double getWidth(char c, int fontSize) {
		if ( c > MAX_CHAR || widths[c]==0 ) return widths['m']/1000.0; // return width('m')
		return widths[c]/1000.0 * fontSize;
	}

	public double getLineHeight(int fontSize) {
		return maxCharHeight / 1000.0 * fontSize;
	}
}