| 1 | "-*-smalltalk-*- |
| 2 | |
| 3 | Smalltalk implementation of a `same-fringe' solver. |
| 4 | |
| 5 | Use GNU Smalltalk syntax -- it seems more Emacs-friendly. |
| 6 | " |
| 7 | |
| 8 | Object subclass: BasicNode [ |
| 9 | <comment: 'I am provide common behaviour for my subclasses Node and |
| 10 | LeafNode. Otherwise, I''m not particularly interesting.'> |
| 11 | |
| 12 | iterator [ |
| 13 | "Return a new iterator to walk this node." |
| 14 | |
| 15 | <category: 'iteration'> |
| 16 | ^NodeIterator for: self |
| 17 | ] |
| 18 | ] |
| 19 | |
| 20 | BasicNode subclass: Node [ |
| 21 | | left right data | |
| 22 | |
| 23 | <comment: 'I represent simple binary tree nodes. My instances consist of |
| 24 | a data object, and left and right subtrees. The leaves of a tree are |
| 25 | instances of LeafNode.'> |
| 26 | <category: 'Toys-SameFringe'> |
| 27 | |
| 28 | Node class >> left: aNode data: anObject right: anotherNode [ |
| 29 | "Answer a newly tree Node with the given subtrees and data." |
| 30 | |
| 31 | <category: 'instance creation'> |
| 32 | ^self new left: aNode data: anObject right: anotherNode |
| 33 | ] |
| 34 | |
| 35 | Node class >> parse: aString [ |
| 36 | "Answer a newly constructed tree, parsed from aString." |
| 37 | |
| 38 | <category: 'parsing'> |
| 39 | | stream tree | |
| 40 | stream := ReadStream on: aString. |
| 41 | tree := stream parseTree. |
| 42 | stream atEnd ifFalse: [self error: 'trailing junk']. |
| 43 | ^tree |
| 44 | ] |
| 45 | |
| 46 | left: aNode data: anObject right: anotherNode [ |
| 47 | "Initialize a (presumably) new instance." |
| 48 | |
| 49 | <category: 'initialization'> |
| 50 | left := aNode. |
| 51 | right := anotherNode. |
| 52 | data := anObject. |
| 53 | ^self |
| 54 | ] |
| 55 | |
| 56 | left [ |
| 57 | "Answer the receiver's left subtree." |
| 58 | |
| 59 | <category: 'accessing'> |
| 60 | ^left |
| 61 | ] |
| 62 | |
| 63 | right [ |
| 64 | "Answer the receiver's right subtree." |
| 65 | |
| 66 | <category: 'accessing'> |
| 67 | ^right |
| 68 | ] |
| 69 | |
| 70 | data [ |
| 71 | "Answer the receiver's data." |
| 72 | |
| 73 | <category: 'accessing'> |
| 74 | ^data |
| 75 | ] |
| 76 | |
| 77 | isLeaf [ |
| 78 | "Answer false, becase the receiver is not a leaf." |
| 79 | |
| 80 | <category: 'testing'> |
| 81 | ^false |
| 82 | ] |
| 83 | |
| 84 | inorderTell: aBlock tell: aNodeIterator [ |
| 85 | "This is the hairy part of the iteration protocol. |
| 86 | |
| 87 | The algorithm works like this. We're meant to wander as far down |
| 88 | the left of the tree as we can; once we're there, we call |
| 89 | aNodeIterator with the data we found and a block which will continue |
| 90 | the iteration over the rest of the tree and finally invoke aBlock. |
| 91 | |
| 92 | Observe that there are no explicit conditionals here. It's all done |
| 93 | with object dispatch. And smoke. And mirrors. |
| 94 | |
| 95 | Also note that this is tail-recursive. The `stack' is built up in |
| 96 | the hairy block constructions, which all go on the heap." |
| 97 | |
| 98 | <category: 'private iteration'> |
| 99 | left |
| 100 | inorderTell: |
| 101 | [aNodeIterator |
| 102 | found: data |
| 103 | then: [right inorderTell: aBlock tell: aNodeIterator]] |
| 104 | tell: aNodeIterator |
| 105 | ] |
| 106 | |
| 107 | sameFringeAs: aNode [ |
| 108 | "Answer whether traversing the receiver inorder yields the same |
| 109 | objects as traversing aNode." |
| 110 | |
| 111 | <category: 'comparison'> |
| 112 | | ia ib | |
| 113 | ia := self iterator. |
| 114 | ib := aNode iterator. |
| 115 | [ia atEnd] whileFalse: |
| 116 | [ib atEnd ifTrue: [^false]. |
| 117 | (ia next = ib next) ifFalse: [^false]]. |
| 118 | ^ib atEnd |
| 119 | ] |
| 120 | |
| 121 | displayOn: aStream [ |
| 122 | "Write a simple representation of self to the stream." |
| 123 | |
| 124 | <category: 'printing'> |
| 125 | aStream nextPut: $(; |
| 126 | display: left; |
| 127 | display: data; |
| 128 | display: right; |
| 129 | nextPut: $) |
| 130 | ] |
| 131 | |
| 132 | Node class >> main: anArray [ |
| 133 | "Noddy script main program." |
| 134 | |
| 135 | <category: 'command line'> |
| 136 | [(Dictionary new |
| 137 | at: 1 put: |
| 138 | [(self parse: (anArray at: 1)) iterator do: |
| 139 | [:char | FileStream stdout nextPut: char]. |
| 140 | FileStream stdout nl]; |
| 141 | at: 2 put: |
| 142 | [FileStream stdout display: |
| 143 | (((self parse: (anArray at: 1)) |
| 144 | sameFringeAs: (self parse: (anArray at: 2))) |
| 145 | ifTrue: ['match'] |
| 146 | ifFalse: ['no match']); |
| 147 | nl ]; |
| 148 | at: anArray size ifAbsent: [self error: 'bad args']) |
| 149 | value] |
| 150 | on: Error do: |
| 151 | [:error | |
| 152 | FileStream stderr |
| 153 | nextPutAll: 'smalltalk-fringe: '; |
| 154 | nextPutAll: error messageText; |
| 155 | nl. |
| 156 | ^1]. |
| 157 | ^0 |
| 158 | ] |
| 159 | ] |
| 160 | |
| 161 | PositionableStream extend [ |
| 162 | parseTree [ |
| 163 | "Answer a newly constructed tree, parsed from the receiver. |
| 164 | |
| 165 | The syntax is very simple: |
| 166 | |
| 167 | tree ::= empty | `(' tree char tree `)' |
| 168 | |
| 169 | where char is any character. Ambiguity is resolved by deciding that |
| 170 | something beginning with `(' where a tree is expected really is a |
| 171 | tree and not an empty tree followed by the char `('." |
| 172 | |
| 173 | <category: 'parsing'> |
| 174 | | left data right | |
| 175 | |
| 176 | self peek = $( ifFalse: [^LeafNode instance]. |
| 177 | self next. |
| 178 | left := self parseTree. |
| 179 | self atEnd ifTrue: [self error: 'no data']. |
| 180 | data := self next. |
| 181 | right := self parseTree. |
| 182 | self next = $) ifFalse: [self error: 'missing )']. |
| 183 | ^Node left: left data: data right: right |
| 184 | ] |
| 185 | ] |
| 186 | |
| 187 | BasicNode subclass: LeafNode [ |
| 188 | <comment: 'I represent the leaves of a tree of Nodes. I don''t hold any |
| 189 | kind of interesting state. My methods provide the base cases for some of the |
| 190 | recursive protocols used to handle Nodes.'> |
| 191 | <category: 'Toys-SameFringe'> |
| 192 | |
| 193 | instance := LeafNode new. |
| 194 | |
| 195 | displayOn: aStream [ |
| 196 | "Write a simple representation of self to the stream." |
| 197 | |
| 198 | <category: 'printing'> |
| 199 | "Nothing to do!" |
| 200 | ] |
| 201 | |
| 202 | isLeaf [ |
| 203 | "Answer true, because the receiver is a leaf node." |
| 204 | |
| 205 | <category: 'testing'> |
| 206 | ^true |
| 207 | ] |
| 208 | |
| 209 | inorderTell: aBlock tell: aNodeIterator [ |
| 210 | "This is the hairy part of the iteration protocol. |
| 211 | |
| 212 | But in this case it's simple. We've overshot the end, so we just |
| 213 | need to call aBlock to persuade our parent to announce itself to the |
| 214 | iterator." |
| 215 | |
| 216 | <category: 'private iteration'> |
| 217 | aBlock value |
| 218 | ] |
| 219 | |
| 220 | LeafNode class >> instance [ |
| 221 | "Return the unique instance of the leaf node." |
| 222 | |
| 223 | <category: 'singleton'> |
| 224 | ^instance |
| 225 | ] |
| 226 | ] |
| 227 | |
| 228 | Stream subclass: NodeIterator [ |
| 229 | | item rest | |
| 230 | |
| 231 | <comment: 'I hold the state for external iteration of trees of Nodes and |
| 232 | (halfheartedly) implement the Stream protocol.'> |
| 233 | <category: 'Toys-SameFringe'> |
| 234 | |
| 235 | found: anObject then: aBlock [ |
| 236 | "Stash the newly found item from the hairy iteration protocol. |
| 237 | |
| 238 | When the iteration protocol decides on the next leftmost item to |
| 239 | return, it gives us anObject that it found, and aBlock which will |
| 240 | continue until it finds the next object." |
| 241 | |
| 242 | <category: 'private iteration'> |
| 243 | item := anObject. |
| 244 | rest := aBlock. |
| 245 | ] |
| 246 | |
| 247 | NodeIterator class >> for: aNode [ |
| 248 | "Answer a new iterator for the tree starting at aNode." |
| 249 | |
| 250 | <category: 'instance creation'> |
| 251 | ^self new walk: aNode |
| 252 | ] |
| 253 | |
| 254 | walk: aNode [ |
| 255 | "Start walking a subtree starting at aNode. |
| 256 | |
| 257 | We get the node to iterate itself and finally tell us that it's |
| 258 | finished." |
| 259 | |
| 260 | <category: 'initialization'> |
| 261 | aNode inorderTell: [rest := nil] tell: self |
| 262 | ] |
| 263 | |
| 264 | next [ |
| 265 | "Answer the next element from the tree, or nil if we've hit the end." |
| 266 | |
| 267 | <category: 'reading'> |
| 268 | | it | |
| 269 | rest ifNil: [^nil]. |
| 270 | it := item. |
| 271 | rest value. |
| 272 | ^it |
| 273 | ] |
| 274 | |
| 275 | peek [ |
| 276 | "Answer the next element without removing it." |
| 277 | |
| 278 | <category: 'reading'> |
| 279 | rest ifNil: [^nil] ifNotNil: [^item] |
| 280 | ] |
| 281 | |
| 282 | atEnd [ |
| 283 | "Answer whether we have reached the end of the iteration." |
| 284 | |
| 285 | <category: 'testing'> |
| 286 | ^rest isNil |
| 287 | ] |
| 288 | ] |