" These macros 'solve' any maze produced by the a-maze-ing maze.c program.
"
" First, a bit of maze theory.
" If you were put into a maze, a guaranteed method of finding your way
" out of the maze is to put your left hand onto a wall and just keep walking,
" never taking your hand off the wall. This technique is only guaranteed to
" work if the maze does not have any 'islands', or if the 'exit' is on the
" same island as your starting point. These conditions hold for the mazes
" under consideration.
"
" Assuming that the maze is made up of horizontal and vertical walls spaced
" one step apart and that you can move either north, south, east or west,
" then you can automate this procedure by carrying out the following steps.
"
" 1. Put yourself somewhere in the maze near a wall.
" 2. Check if you have a wall on your left. If so, go to step 4.
" 3. There is no wall on your left, so turn on the spot to your left and step
"    forward by one step and repeat step 2.
" 4. Check what is directly in front of you. If it is a wall, turn on the
"    spot to your right by 90 degrees and repeat step 4.
" 5. There is no wall in front of you, so step forward one step and
"    go to step 2.
"
" In this way you will cover all the corridors of the maze (until you get back
" to where you started from, if you do not stop).
"
" By examining a maze produced by the maze.c program you will see that
" each square of the maze is one character high and two characters wide.
" To go north or south, you move by a one character step, but to move east or
" west you move by a two character step. Also note that in any position
" there are four places where walls could be put - to the north, to the south,
" to the east and to the west.
" A wall exists to the north of you if the character to the north of
" you is a _ (otherwise it is a space).
" A wall exists to the east of you if the character to the east of you
" is a | (otherwise it is a .).
" A wall exists to the west of you if the character to the west of you
" is a | (otherwise it is a .).
" A wall exists to the south of you if the character where you are
" is a _ (otherwise it is a space).
"
" Note the difference for direction south, where we must examine the character
" where the cursor is rather than an adjacent cell.
"
" If you were implementing the above procedure is a normal computer language
" you could use a loop with if statements and continue statements,
" However, these constructs are not available in vi macros so I have used
" a state machine with 8 states. Each state signifies the direction you
" are going in and whether or not you have checked if there is a wall on
" your left.
"
" The transition from state to state and the actions taken on each transition
" are given in the state table below.
" The names of the states are N1, N2, S1, S2, E1, E2, W1, W2, where each letter
" stands for a direction of the compass, the number 1 indicates that the we
" have not yet checked to see if there is a wall on our left and the number 2
" indicates that we have checked and there is a wall on our left.
"
" For each state we must consider the existence or not of a wall in a
" particular direction. This direction is given in the following table.
"
" NextChar table:
" state        direction       vi commands
"  N1              W               hF
"  N2              N               kF
"  S1              E               lF
"  S2              S               F
"  E1              N               kF
"  E2              E               lF
"  W1              S               F
"  W2              W               hF
"
" where F is a macro which yanks the character under the cursor into
" the NextChar register (n).
"
" State table:
" In the 'vi commands' column is given the actions to carry out when in
" this state and the NextChar is as given. The commands k, j, ll, hh move
" the current position north, south, east and west respectively. The
" command mm is used as a no-op command.
" In the 'next state' column is given the new state of the machine after
" the action is carried out.
"
" current state        NextChar    vi commands  next state
"      N1                 .            hh          W1
"      N1                 |            mm          N2
"      N2                 _            mm          E1
"      N2               space          k           N1
"      S1                 .            ll          E1
"      S1                 |            mm          S2
"      S2                 _            mm          W1
"      S2               space          j           S1
"      E1               space          k           N1
"      E1                 _            mm          E2
"      E2                 |            mm          S1
"      E2                 .            ll          E1
"      W1               space          j           S1
"      W1                 _            mm          W2
"      W2                 |            mm          N1
"      W2                 .            hh          W1
"
"
" Complaint about vi macros:
" It seems that you cannot have more than one 'undo-able' vi command
" in the one macro, so you have to make lots of little macros and
" put them together.
"
" I'll explain what I mean by an example. Edit a file and
" type ':map Q rXY'. This should map the Q key to 'replace the
" character under the cursor with X and yank the line'.
" But when I type Q, vi tells me 'Can't yank inside global/macro' and
" goes into ex mode. However if I type ':map Q rXT' and ':map T Y',
" everything is OK. I`m doing all this on a Sparcstation.
" If anyone reading this has an answer to this problem, the author would
" love to find out. Mail to gregm@otc.otca.oz.au.
"
" The macros:
" The macro to run the maze solver is 'g'. This simply calls two other
" macros: I, to initialise everything, and L, to loop forever running
" through the state table.
" Both of these macros are long sequences of calls to other macros. All
" of these other macros are quite simple and so to understand how this
" works, all you need to do is examine macros I and L and learn what they
" do (a simple sequence of vi actions) and how L loops (by calling U, which
" simply calls L again).
"
" Macro I sets up the state table and NextChar table at the end of the file.
" Macro L then searches these tables to find out what actions to perform and
" what state changes to make.
"
" The entries in the state table all begin with a key consisting of the
" letter 's', the current state and the NextChar.  After this is the
" action to take in this state and after this is the next state to change to.
"
" The entries in the NextChar table begin with a key consisting of the
" letter 'n' and the current state. After this is the action to take to
" obtain NextChar - the character that must be examined to change state.
"
" One way to see what each part of the macros is doing is to type in the
" body of the macros I and L manually (instead of typing 'g') and see
" what happens at each step.
"
" Good luck.
"
" Registers used by the macros:
" s (State)        - holds the state the machine is in
" c (Char)         - holds the character under the current position
" m (Macro)        - holds a vi command string to be executed later
" n (NextChar)     - holds the character we must examine to change state
" r (Second Macro) - holds a second vi command string to be executed later
"
set remap
set nomagic
set noterse
set wrapscan
"
"================================================================
" g - go runs the whole show
"        I - initialise
"        L - then loop forever
map g   IL
"
"================================================================
" I - initialise everything before running the loop
"   G$?.^M - find the last . in the maze
"        ^ - replace it with an X (the goal)
"   GYKeDP - print the state table and next char table at the end of the file
"       0S - initialise the state of the machine to E1
"      2Gl - move to the top left cell of the maze
map I   G$?.
^GYKeDP0S2Gl
"
"================================================================
" L - the loop which is executed forever
"        Q - save the current character in the Char register
"        A - replace the current character with an 'O'
"       ma - mark the current position with mark 'a'
"      GNB - on bottom line, create a command to search the NextChar table
"            for the current state
" 0M0E@m^M - yank the command into the Macro register and execute it
"       wX - we have now found the entry in the table, now yank the
"            following word into the Macro register
"     `a@m - go back to the current position and execute the macro, this will
"            yank the NextChar in register n
"   GT$B$R - on bottom line, create a command to search the state table
"            for the current state and NextChar
" 0M0E@m^M - yank the command into the Macro register and execute it
"      2WS - we have now found the entry in the table, now yank the
"            next state into the State macro
"       bX - and yank the action corresponding to this state table entry
"            into the Macro register
"      GVJ - on bottom line, create a command to restore the current character
"       0H - and save the command into the second Macro register
"     `a@r - go back to the current position and exectute the macro to restore
"            the current character
"       @m - execute the action associated with this state
"        U - and repeat
map L   QAmaGNB0M0E@m
wX`a@mGT$B$R0M0E@m
2WSbXGVJ0H`a@r@mU
"
"================================================================
" U - no tail recursion allowed in vi macros so cheat and set U = L
map U   L
"
"================================================================
" S - yank the next two characters into the State register
map S   "sy2l
"
"================================================================
" Q - save the current character in the Char register
map Q   "cyl
"
"================================================================
" A - replace the current character with an 'O'
map A   rO
"
"================================================================
" N - replace this line with the string 'n'
map N   C/n
"
"================================================================
" B - put the current state
map B   "sp
"
"================================================================
" M - yank this line into the Macro register
map M   "my$
"
"================================================================
" E - delete to the end of the line
map E   d$
"
"================================================================
" X - yank this word into the Macro register
map X   "myt 
"
"================================================================
" T - replace this line with the string 's'
map T   C/s
"
"================================================================
" R - put NextChar
map R   "np
"
"================================================================
" V - add the letter 'r' (the replace vi command)
map V   ar
"
"================================================================
" J - restore the current character
map J   "cp
"
"================================================================
" H - yank this line into the second Macro register
map H   "ry$
"
"================================================================
" F - yank NextChar (this macro is called from the Macro register)
map F   "nyl
"
"================================================================
" ^ - replace the current character with an 'X'
map ^   rX
"
"================================================================
" YKeDP - create the state table, NextChar table and initial state
" Note that you have to escape the bar character, since it is special to
" the map command (it indicates a new line).
map Y   osE1  k  N1       sE1_ mm E2       sE2| mm S1       sE2. ll E1
map K   osW1  j  S1       sW1_ mm W2       sW2| mm N1       sW2. hh W1
map e   osN1. hh W1       sN1| mm N2       sN2  k  N1       sN2_ mm E1
map D   osS1. ll E1       sS1| mm S2       sS2  j  S1       sS2_ mm W1
map P   onE1 kF nE2 lF nW1 G$JF nW2 hF nN1 hF nN2 kF nS1 lF nS2 G$JF 
E1