Hello world and puzzle program.

combinations/Makefile

@@ -0,0 +1,65 @@
+CC := gcc
+CXX := g++
+OPTIMIZATION := -O3
+CFLAGS = ${OPTIMIZATION} -Werror -Wextra -Wall -Wpedantic
+# all: helloworld
+#
+# #automatically built from implicit rules.
+# helloworld: helloworld.c
+#
+# clean:
+# 	rm -f helloworld
+#
+TARGET_EXEC := combinations
+
+BUILD_DIR := ./build
+SRC_DIRS := ./src
+
+# Find all the C and C++ files we want to compile
+# Note the single quotes around the * expressions. The shell will incorrectly expand these otherwise, but we want to send the * directly to the find command.
+SRCS := $(shell find $(SRC_DIRS) -name '*.cpp' -or -name '*.c' -or -name '*.s')
+
+# Prepends BUILD_DIR and appends .o to every src file
+# As an example, ./your_dir/hello.cpp turns into ./build/./your_dir/hello.cpp.o
+OBJS := $(SRCS:%=$(BUILD_DIR)/%.o)
+
+# String substitution (suffix version without %).
+# As an example, ./build/hello.cpp.o turns into ./build/hello.cpp.d
+DEPS := $(OBJS:.o=.d)
+
+# Every folder in ./src will need to be passed to GCC so that it can find header files
+INC_DIRS := $(shell find $(SRC_DIRS) -type d)
+# Add a prefix to INC_DIRS. So moduleA would become -ImoduleA. GCC understands this -I flag
+INC_FLAGS := $(addprefix -I,$(INC_DIRS))
+
+# The -MMD and -MP flags together generate Makefiles for us!
+# These files will have .d instead of .o as the output.
+DEPFLAGS := $(INC_FLAGS) -MMD -MP
+
+# The final build step.
+$(BUILD_DIR)/$(TARGET_EXEC): $(OBJS)
+	$(CXX) $(OBJS) -o $@ $(LDFLAGS)
+
+# Build step for C source
+$(BUILD_DIR)/%.c.o: %.c
+	mkdir -p $(dir $@)
+	$(CC) $(DEPFLAGS) $(CFLAGS) -c $< -o $@
+
+# Build step for C++ source
+$(BUILD_DIR)/%.cpp.o: %.cpp
+	mkdir -p $(dir $@)
+	$(CXX) $(DEPFLAGS) $(CXXFLAGS) -c $< -o $@
+
+
+.PHONY: clean
+clean:
+	rm -r $(BUILD_DIR)
+
+.PHONY: run
+run: $(BUILD_DIR)/$(TARGET_EXEC)
+	$(BUILD_DIR)/$(TARGET_EXEC)
+
+# Include the .d makefiles. The - at the front suppresses the errors of missing
+# Makefiles. Initially, all the .d files will be missing, and we don't want those
+# errors to show up.
+-include $(DEPS)

combinations/src/combinations.c

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+/* calculates all subsets of the string comb by assigning eatch letter a position in a binary number and counting up. */
+#include <stdio.h>
+#include <string.h>
+
+// #define DEBUG
+
+int main() {
+  char* comb = "ABCD";
+  int count = strlen(comb);
+  // that bitshifting is the same is squaring count
+  int permutations = (1 << count);
+
+  printf("string is %s\nthere are %d permutations", comb, permutations);
+
+  for (unsigned char i = 0; i < permutations; i++) {
+    for (unsigned char j = 0; j < 4; j++) {
+      // when j=0, will result in 0b0001, when j=1, 0b0010, j=2 0b0100.
+      unsigned char mask = 1 << j;
+#ifdef DEBUG
+      printf(" j=%x mask=%x, postmask=%x, i=%x", j, mask, (i & mask), i);
+#endif /* ifdef DEBUG */
+      if ((i & mask) != 0) {
+        printf(" %c", comb[j]);
+      }
+    }
+    printf("\n");
+  }
+
+  return 0;
+}