In a previous log I said, "I'm not going to overthink this. Random selection from the chess openings is working.". Well I lied. I am going to overthink this. (Well at least think it.)

Something I should have done when working on the opening moves was to actually see what they were. So I dumped the 32 chess openings in a format I could read.  Here is the Python script I used:

col_table = ['H','G','F','E','D','C','B','A']
row_table = ['1','2','3','4','5','6','7','8']

with open("Opening Book.bin", 'rb') as f:
    buffer = f.read()
   
    # For each line.
    ent_file = []
    even = 0
    address = 0x8C00
    for i in range(0,len(buffer),2):
        if i % 32 == 0:
            ent_file.append("\n")
            ent_file.append(hex(address))
            ent_file.append(": ")
            address += 32
        # Assume bytes are contiguous.
        if (even % 2) == 0:
            ent_file.append(col_table[buffer[i]&0x0F]+row_table[buffer[i]>>4]+"-"+
                            col_table[buffer[i+1]&0x0F]+row_table[buffer[i+1]>>4] +", ")   
        else:
            ent_file.append(col_table[7-(buffer[i]&0x0F)]+row_table[7-(buffer[i]>>4)]+"-"+
                            col_table[7-(buffer[i+1]&0x0F)]+row_table[7-(buffer[i+1]>>4)] +", ")
        even += 1     
    ent_file.append('\n')
    
    # Output the result.
    with open("Opening Book Dump.txt", 'w') as f:
        f.write(''.join(ent_file))

It wasn't to hard to figure out the format.

8c00: E2-E4, E7-E5, G1-F3, B8-C6, B1-C3, G8-F6, F1-B5, F8-B4, E1-G1, E8-G8, D2-D3, D7-D6, C1-G5, B4-C3, B2-C3, D8-E7
8c20: D2-D4, D7-D5, C2-C4, D5-C4, G1-F3, G8-F6, E2-E3, E7-E6, F1-C4, C7-C5, E1-G1, A7-A6, D1-E2, B8-C6, B1-C3, C5-D4 
8c40: F2-F4, D7-D5, E2-E3, G8-F6, G1-F3, C7-C5, B2-B3, E7-E6, C1-B2, B8-C6, F1-B5, C8-D7, E1-G1, F8-D6, D2-D3, D8-C7 
8c60: E2-E4, E7-E5, F1-C4, G8-F6, D2-D4, E5-D4, G1-F3, F6-E4, D1-D4, E4-F6, C1-G5, F8-E7, B1-C3, C7-C6, E1-C1, D7-D5 
8c80: E2-E4, C7-C5, G1-F3, B8-C6, D2-D4, C5-D4, F3-D4, G8-F6, B1-C3, D7-D6, F1-E2, G7-G6, C1-E3, F8-G7, E1-G1, E8-G8 
8ca0: E2-E4, C7-C5, B1-C3, B8-C6, G2-G3, G7-G6, F1-G2, F8-G7, D2-D3, E7-E6, C1-E3, D7-D6, G1-E2, C6-D4, E1-G1, G8-E7
8cc0: E2-E4, E7-E5, G1-F3, B8-C6, F1-C4, F8-C5, C2-C3, G8-F6, D2-D4, E5-D4, C3-D4, C5-B4, B1-C3, F6-E4, E1-G1, E4-C3 
8ce0: G1-F3, G8-F6, C2-C4, C7-C5, D2-D4, C5-D4, F3-D4, E7-E6, B1-C3, F8-B4, C1-D2, E8-G8, E2-E3, B8-C6, F1-E2, D7-D5 
8d00: E2-E4, E7-E5, G1-F3, G8-F6, F3-E5, D7-D6, E5-F3, F6-E4, D2-D4, D6-D5, F1-D3, F8-D6, E1-G1, E8-G8, C2-C4, C7-C6 
8d20: D2-D4, G8-F6, C2-C4, E7-E6, G2-G3, D7-D5, F1-G2, D5-C4, D1-A4, B8-D7, A4-C4, A7-A6, G1-F3, B7-B5, C4-C6, A8-A7
8d40: E2-E4, G8-F6, E4-E5, F6-D5, D2-D4, D7-D6, C2-C4, D5-B6, F2-F4, D6-E5, F4-E5, B8-C6, C1-E3, C8-F5, B1-C3, E7-E6 
8d60: D2-D4, F7-F5, C2-C4, E7-E6, G1-F3, G8-F6, G2-G3, F8-E7, F1-G2, E8-G8, E1-G1, D7-D5, B1-C3, C7-C6, C1-F4, D8-E8 
8d80: E2-E4, E7-E5, G1-F3, B8-C6, F1-C4, G8-F6, D2-D4, E5-D4, E1-G1, F6-E4, F1-E1, D7-D5, C4-D5, D8-D5, B1-C3, D5-A5 
8da0: D2-D4, G8-F6, C2-C4, E7-E6, B1-C3, F8-B4, D1-C2, B8-C6, G1-F3, D7-D6, C1-D2, E6-E5, A2-A3, B4-C3, D2-C3, D8-E7 
8dc0: E2-E4, B8-C6, D2-D4, D7-D5, E4-D5, D8-D5, G1-F3, E7-E5, B1-C3, F8-B4, C1-E3, C8-G4, F1-E2, E8-C8, E1-G1, D5-A5 
8de0: D2-D4, D7-D5, C2-C4, E7-E6, B1-C3, G8-F6, C1-G5, B8-D7, G1-F3, F8-B4, C4-D5, E6-D5, E2-E3, C7-C5, F1-D3, D8-A5
8e00: D2-D4, D7-D5, C2-C4, C7-C6, G1-F3, G8-F6, B1-C3, D5-C4, A2-A4, C8-F5, F3-E5, B8-D7, E5-C4, D8-C7, G2-G3, E7-E5
8e20: E2-E4, E7-E5, G1-F3, B8-C6, F1-B5, D7-D6, D2-D4, C8-D7, B1-C3, G8-F6, E1-G1, F8-E7, F1-E1, E5-D4, F3-D4, E8-G8 
8e40: D2-D4, D7-D5, C2-C4, E7-E6, B1-C3, C7-C5, C4-D5, E6-D5, G1-F3, B8-C6, G2-G3, G8-F6, F1-G2, C5-D4, F3-D4, F8-C5 
8e60: E2-E4, E7-E5, G1-F3, B8-C6, F1-B5, A7-A6, B5-C6, D7-C6, D2-D4, E5-D4, D1-D4, D8-D4, F3-D4, C8-D7, B1-C3, E8-C8 
8e80: D2-D4, C7-C5, D4-D5, D7-D6, C2-C4, G7-G6, B1-C3, F8-G7, E2-E4, G8-F6, F1-E2, E7-E6, C1-G5, E8-G8, G1-F3, E6-D5 
8ea0: E2-E4, E7-E5, G1-F3, B8-C6, D2-D4, E5-D4, F3-D4, G8-F6, B1-C3, F8-B4, D4-C6, B7-C6, F1-D3, D7-D5, E4-D5, C6-D5 
8ec0: D2-D4, G8-F6, C2-C4, G7-G6, B1-C3, F8-G7, E2-E4, D7-D6, F2-F3, E7-E5, D4-D5, E8-G8, C1-G5, H7-H6, G5-E3, F6-H5 
8ee0: E2-E4, C7-C6, D2-D4, D7-D5, B1-C3, D5-E4, C3-E4, C8-F5, E4-G3, F5-G6, H2-H4, H7-H6, G1-F3, B8-D7, F1-D3, G6-D3 
8f00: D2-D4, G8-F6, C2-C4, E7-E6, G1-F3, B7-B6, G2-G3, C8-B7, F1-G2, F8-E7, E1-G1, E8-G8, B1-C3, F6-E4, D1-C2, E4-C3 
8f20: C2-C4, G8-F6, B1-C3, E7-E6, E2-E4, C7-C5, G1-F3, B8-C6, D2-D4, C5-D4, F3-D4, F8-B4, D4-C6, D7-C6, D1-D8, E8-D8 
8f40: E2-E4, E7-E5, G1-F3, B8-C6, F1-B5, G8-F6, E1-G1, F6-E4, D2-D4, F8-E7, D1-E2, E4-D6, B5-C6, B7-C6, D4-E5, D6-B7 
8f60: E2-E4, E7-E6, D2-D4, D7-D5, B1-C3, G8-F6, C1-G5, F8-E7, E4-E5, F6-D7, G5-E7, D8-E7, D1-D2, E8-G8, F2-F4, C7-C5
8f80: E2-E4, E7-E5, D2-D4, E5-D4, D1-D4, B8-C6, D4-E3, G8-F6, B1-C3, F8-B4, C1-D2, E8-G8, E1-C1, F8-E8, F1-C4, D7-D6 
8fa0: E2-E4, E7-E5, D2-D4, E5-D4, C2-C3, D4-C3, F1-C4, C3-B2, C1-B2, G8-F6, B1-C3, B8-C6, G1-F3, F8-B4, D1-C2, D7-D6 
8fc0: C2-C4, E7-E5, B1-C3, G8-F6, G1-F3, B8-C6, E2-E3, D7-D5, C4-D5, F6-D5, F1-B5, D5-C3, B2-C3, F8-D6, D2-D4, C8-D7 
8fe0: E2-E4, E7-E5, G1-F3, B8-C6, F1-B5, A7-A6, B5-A4, B7-B5, A4-B3, C6-A5, B3-F7, E8-F7, F3-E5, F7-E7, D2-D4, G8-F6

The first thing that pops out is that 28 of the 32 openings start with either D2-D4 (10) or E2-E4 (18). That explains the very high percentage of the time that these occur when openings are randomly selected. 

The Operation Instructions explicitly states that CHESSmate chooses one opening at random and tries to follow it for 16 moves. I verified that this is the case by playing a few games and using the table above to pick moves in and out of the opening chosen. Dropping out of the opening book when the first difference is encountered might be a missed opportunity. Many of the E4 and D4 openings have the same first moves before diverging. A better strategy might have been to search the other openings, when a move diverges from the current opening, and switch a new opening if one can be found with the same previous moves and the next move that matches the new move.

That feels like a lot of new code, and I don't see a lot of unused space in the ROM, so the simpler solution might be all that would fit. Fun to think about.

Another thought. Knowing the format of the opening book would allow someone to replace the openings with their own. Might be cool to try. Are there more "modern" openings that could be substituted? Maybe a TODO.