From f50cecf2cb77cc073cb86a6016468a09d1c49fb0 Mon Sep 17 00:00:00 2001
From: Ben Sima <ben@bsima.me>
Date: Sat, 23 Mar 2019 23:58:26 -0700
Subject: Add semi-working bs

---
 lore/sicp/meta.scm | 20 ++++++++++++++++++++
 lore/sicp/set.scm  | 42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+)
 create mode 100644 lore/sicp/meta.scm
 create mode 100644 lore/sicp/set.scm

(limited to 'lore/sicp')

diff --git a/lore/sicp/meta.scm b/lore/sicp/meta.scm
new file mode 100644
index 0000000..94dc784
--- /dev/null
+++ b/lore/sicp/meta.scm
@@ -0,0 +1,20 @@
+(define (eval exp env)
+  (cond
+    ((self-evaluating? exp) exp)
+    ((variable? exp) (lookup-variable-value exp env))
+    ((quoted? exp) (text-of-quotation exp))
+    ((assignment? exp) (eval-assignment exp env))
+    ((definition? exp) (eval-definition exp env))
+    ((if? exp) (eval-if exp env))
+    ((lambda? exp)
+     (make-procedure (lambda-parameters exp)
+                     (lambda-body exp)
+                     env))
+    ((begin? exp)
+     (eval-sequence (begin-actions exp) env))
+    ((cond? exp) (eval (cond->if exp) env))
+    ((application? exp)
+     (apply (eval (operator exp) env)
+            (list-of-values (operands exp) env)))
+    (else
+      (error "Unknown expression type -- EVAL" exp))))
diff --git a/lore/sicp/set.scm b/lore/sicp/set.scm
new file mode 100644
index 0000000..3d60c5c
--- /dev/null
+++ b/lore/sicp/set.scm
@@ -0,0 +1,42 @@
+;; A set module, based on binary trees (from sicp)
+
+(define (entry tree) (car tree))
+(define (left-branch tree) (cadr tree))
+(define (right-branch tree) (caddr tree))
+
+(define (make-tree entry left right)
+  (list entry left right))
+
+(define (element-of-set? x set)
+  (cond ((null? set) #f)
+        ((= x (entry set)) #t)
+        ((< x (entry set))
+         (element-of-set? x (left-branch set)))
+        ((> x (entry set))
+         (element-of-set? x (right-branch set)))))
+
+(define (adjoin-set x set)
+  (cond ((null? set) (make-tree x '() '()))
+        ((= x (entry set)) set)
+        ((< x (entry set))
+         (make-tree (entry set)
+                    (adjoin-set x (left-branch set))
+                    (right-branch set)))
+        ((> x (entry set))
+         (make-tree (entry set)
+                    (left-branch set)
+                    (adjoin-set x (right-branch set))))))
+
+(define (intersection-set set1 set2)
+  (if (or (null? set1) (null? set2))
+    '()
+    (let ((x1 (car set1))
+          (x2 (car set2)))
+      (cond ((= x1 x2)
+             (cons x1
+                   (intersection-set (cdr set1)
+                                     (cdr set2))))
+            ((< x1 x2)
+             (intersection-set (cdr set1) set2))
+            ((< x2 x1)
+             (intersection-set set1 (cdr set2)))))))
-- 
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