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    <p>I'm going to make some assumptions here: The board is stored as a 64 bit number, each 8 byte block represents a row. Each bit in the row represents a column (a..h). You have start and end position as zero-based coordinates. i.e: <em><code>start = "C4" = [2,3]; end = "F4" = [5,3]</code></em></p> <p>For horizontal moves with increasing columns, you can calculate the distance moved: <em><code>d = (F4-C4 = 3)</code></em>. Subtract 1 to exclude the destination, then the "trail" t of d-1 bits is <em><code>t = (1&lt;&lt;(d-1))-1</code></em>. Shift the trail adjacent to the source piece to get the mask M: <em><code>M = t&lt;&lt;(start.row*8 + start.column+1)</code>.</em></p> <p>This is equivalent to <code>M = ((1&lt;&lt;d)-2)&lt;&lt;(start.row*8 + start.column)</code></p> <p>For horizontal moves the other way: </p> <pre><code> d = (C4-F4 = -3) t = (1&lt;&lt;(-d-1))-1 M = (t&lt;&lt;dest.column+1) //-or- M = ((1&lt;&lt;-d)-2)&lt;&lt;(dest.row*8 + dest.column) </code></pre> <p>For vertically increasing moves: </p> <pre><code> d = (C7-C4 = 3) t=(1&lt;&lt;8) (d-1) times: { t |= (t&lt;&lt;8)} M = t &lt;&lt; (start.row*8 + start.column) </code></pre> <p>For vertically decreasing moves: </p> <pre><code> d = (C4-C7 = 3) t=(1&lt;&lt;8) (d-1) times: { t |= (t&lt;&lt;8)} M = t &lt;&lt; (dest.row*8 + start.column) </code></pre> <p>For the vertical moves, you can replace the loop over d by storing the maximum "vertical trail" <code>VT = 0x0101010101010101 = 72340172838076673</code>. Then mask out the right number of bits for the actual move. </p> <p>This reduces the caclulation to <code>M = (VT &amp; ((1&lt;&lt;(d*8)) - 2)) &lt;&lt; (row*8+column)</code>.</p> <p>You could probably do something similar for the diagonal moves. Start with the max diagonal trail DT = <code>0x0102040810204080</code>, apply a mask to reduce it to <code>d</code> set bits, and shift to the start or end location, depending on which is closer to the edge. This would need careful testing to make sure there were no edge cases which wrapped around into the wrong row.</p> <p><em>Edited to exclude both source and destination, and fix one-off errors</em></p>
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    1. POGetting occupancy bit masks for bitboards
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    1. POGetting occupancy bit masks for bitboards
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    1. COthank you for your answer! I will try this out tonight and post the results.
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    2. COOne question: why do it this way ? Is this only for human player moves? For AI moves, it would seem easier to calculate the non-blocked moves first, then pick the best one, rather than picking a move out of all the possible ones and then testing if it is blocked.
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    3. COI'm doing it for validation of moves written in a game. In certain notations you have to figure out the source square of the piece that moved to a destination. For example, in some notation you might get just Rf4 for a move of rook from c4 to f4. And if you have more than 1 rook, you will have to use validation to find out which one can move to f4. Anyway, with some minor tweaks to your formulas, I got the right bitmasks for the squares in between 2 given. Will benchmark and post the results soon. Thank you!
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