diff --git a/day_6/Makefile b/day_6/Makefile
new file mode 100644
index 0000000..81b7c3f
--- /dev/null
+++ b/day_6/Makefile
@@ -0,0 +1,56 @@
+CC := clang
+YACC := bison
+LEX := flex
+CFLAGS := -std=c17 -Wpedantic -Wall -Wextra
+BISONFLAGS := 
+BUILD := ./build
+INCLUDE	:= -I./
+OBJ_DIR := $(BUILD)/objects
+APP_DIR := $(BUILD)/bin
+TARGET := day_6
+
+SRC := $(wildcard ./*.cpp) lexer.c parser.c
+
+OBJECTS	:= $(SRC:%.c=$(OBJ_DIR)/%.o)
+
+# targets
+all: build $(APP_DIR)/$(TARGET)
+
+run: all
+	$(APP_DIR)/$(TARGET)
+
+flex:
+	$(LEX) --outfile=lexer.c --header-file=lexer.h lexer.l
+
+bison:
+	$(YACC) $(BISONFLAGS) --output=parser.c parser.y
+
+generate: flex bison
+
+$(OBJ_DIR)/%.o: %.c
+	@mkdir -p $(@D)
+	$(CC) $(CFLAGS) $(INCLUDE) -o $@ -c $<
+
+$(APP_DIR)/$(TARGET): flex bison $(OBJECTS)
+	@mkdir -p $(@D)
+	$(CC) $(CFLAGS) $(INCLUDE) $(LDFLAGS) -o $(APP_DIR)/$(TARGET) $(OBJECTS)
+
+.PHONY: all build clean debug release
+
+build:
+	@mkdir -p $(APP_DIR)
+	@mkdir -p $(OBJ_DIR)
+
+debug: CFLAGS += -DDEBUG -g
+debug: BISONFLAGS += -Wcounterexamples
+debug: all
+
+sanitize: CFLAGS += -DDEBUG -g -fsanitize=address
+sanitize: all
+
+release: CFLAGS += -O2
+release: all
+
+clean:
+	-@rm -rvf $(BUILD)
+	-@rm -rvf {lexer.c,parser.c} {lexer.h,parser.h}
diff --git a/day_6/input.txt b/day_6/input.txt
new file mode 100644
index 0000000..65120b7
--- /dev/null
+++ b/day_6/input.txt
@@ -0,0 +1,2 @@
+hrbbjllllspssblslvvrdrbbpbbmcccfppvbbwvbbmrmjrjrfrgfgbffgfqfqlltlwttscsncscchssrppffvwwvvpnnwwwpvwvhhnvhhbttvzzdlzdlzzwmmjhhznnjdnnnqddbtdbdbsdsmdsdrrdpdwpdppgcgqgcctftsfszslljbljbjwbwbnwnqqrnnztntmtrmmzwzdwzwgwwwjhjsjgjtjjhpjhhppqzqdqffrvrtvvsmmgwmgwgbbclltctptzpzhpzptzppcfpcfftflfzftztddzgzmmfsmsrmmsstttvbvmbvvsmsqmqlldjdtthwtwbwggrzzjrzrcctffsshqshhpthhlnhlhqhdqqrwrmmcttpfttzfzgzdgdzzwrrtsrrsnrnccrbbsssbpbjjvzzwlwtwjwsjwjggzqgzzrsszzjnzjzwjzzcrzczncnqqztzfzhfhvvtjvjdvjjmrjrppvzppczpczcggshghvhnhhsrsnsdszzdpzpzlpzzhwwmnwmwmcwwfnwfwjjcbcncllcsllqdqzzhqhmqmbbjvjjwwcjjpnpllzfzddtmtccqrcrtrwrpphpmpplslmltthnnvhhvrvbblhhrrdqqmbqqgtqggdgcdcvvsbsswvvpggbbtftlflglzlmlbbfhhrshswhshffhhdnnrfrvrmrnrprrmfmpmnpnfnggvvcncdcrczrzccpmmssrbbdjdtdrdwrrwhrrvrtvvszvvwvzzmhhjhhwlhlqlvlttzftztdtstftrfrdddmtmzzsqzqvvpdpdcpcncnrrtntznzrzgznztnznhhsqqnrqrhhlzhzthhfddrzdrrqmqggcmmllnjjvwwjccfjfqfcfzccwvcwvcvjcjtjtnnqsqmqrmrzrszrszzfwfggnmmcdmdjmmhwwgfwfnwnlwwcffsrffvnvbvnvwnwgnwnmmbzbmbpplmplpspzpmzpzdzgzrzrtrjrbbppwvwgwmggqwgghqqshhcwcqwcqcfcbbnsnrrtztzrtrvtvhthzhmmrqqrwqqsjqjcctgtwthhqmmnffmgmdgdlgljjhwjwggrqqfrqqjvqvhqqgsqsgsttmrrbprbppmjppslpsllvlfvlvrvhvtqmrjcdzwsbzfmgmwmwqwhztqrsdzhqjqvbjbntnbndflthljcczdmmhszfgsplrtlqnfzbrlqngwdqtfwcmrdjrsmdpmjmqwrbwfjzwnvqhfmlqtvvnlfzbfccwslqpbzzjccbvrzhghqwtvqgwrmsfzqnmnqqjsjtpcmngpqgllfsnpqtjjbqcdppnsmtwrslnrbqtwvnbctzvwfmgctscmzjbqqgqdwbpzmrdwgfcjzftzgmfcjhchbnmnqnrgtqngwrmncjvptqqdtjtgtpzzdrfsdgmwlwrjnqldbwrqjrhwcczlzvlhpgrnwzhbwjnpthggczfgtrjnzvnlfdfbwcnzfbwlwlmgnnjnpvhbhqgnzhqsnmvbcftsmrcgpvnnnmgnrvpbzlpwnbwpzmwpgqvbfgjwfrjqnvvgmqwwcfddqmdznmfhpjcfgptqdqwmplrglbwlmsqzjshrlhflcjvptgrcfhjfgqmlfzrtphpbvcqzwpcnwljjdlmqzhcctqshdngrgtlfsrfccdtlvmqcdgnpcvphdsrpzfzwclvsqcpzqlfvvqzggdhpfzdvhshglvfzfmcllrdfjfsjtngjgddcpqnlmrnplwtlvwdvzftltnsnspcdztgqhlhvvbnwvnmhscfnqbngpvprzfrjcmfpfzfftrlnwgllhnjndpjdrwcgqpcgcqngnbfzlvzvhnqdjthflmwvppmbdssddmgsbgrqnpjzrjpzdddqgsdlmwnhhpjbthclvqhgrsnrbqgtnsjhncnzbhrdgftvbptrqssvsqfpqnddhmgwcrfqndqjsqgffmhdvqhjrdlmrlcqctqccprwlbqgqrwmtfhwmfjfqzdqbsdsjbtsvfvgbsrvqwnqqqqthpsqgcfslsqtnjwtsrcdcctggdghrjwpbfccrtwgszwbrsjswmjmjbcqrsgbcfsdjzsbjnnssnddnnvwgftlrqvphnqcgjszscrlhhjnljlqcjqtqfwbmdmrgdlcqqwmbsmsdhpplvlfglqwspbfptlbzqjwhqmfvzvsvpjclcdzsbvntmhdqdvhghcmmflpjbglsghbswdshtsbdrgpsrsclrmfwwqbrgdjsqztgttqpwhnfhszlgbfpzhczsnwqflmshlgbrpmdzgpqwtsbssgfjbtrwbmztlwwfmsdgpgfgdjfdccwlfgztbcbqjvjtvslmddjplrswwcszspgplsrhrnwnmrrfbcgdmntcrlvnfqtwwcczsglrhtrfqnmhvgzjpmlplqvqhmnfgvzqcmzhqszgslvndqtqhvrbvbmclbcbjdswvcjrzgfdmdwnnlzlzqcffsrqdfmmpzfnmdsnqlpcrhzsdnsflblcjsfsgcnsspftjrlmdjsmfpqtmlgfvnlfnjscsgwzwvpjrvvclhsbqldlnmtglhbjfwlzmvrbvgtprfjbjhhnlqnbrswwlqtcgrjrltdrnfrjhrntllptlsbhqrwvdsfrlghtfcndznzjwcgmtdvffltgrdmljlqhdtmdvnfsfsrvdpmhlrrsttvqlwfptddwbpfrbclwwzmfpttmrmmqzjnbbnnfvzwmmcfshvrlbdbjzprftbqvdsghnnzwbjccpthdsvsdlgvphsgjdqjwsgmzqnqpqvgqjvwgjtzpmqqwnlwrwhqqjjclcbhjgpwhqdclwmqfmwbwmwwvcbhfznfhcfbprfcdqlbcttnvgnjwswcmpbrghtzgdbppbprffzjgvddzpwmdctrhnrfzdfhtmnfrsfdqvzcnrtncflhvldcndwqtvbggmwlzhchlcwtcbqcvlfhdwljgddwpvcfczvfqmphgtdsnsqwdpvvmwnwqjbrjwbdhhgtffphsdrvspsbgmfrmwmhnrgqdfppzgfpgmqjcsnglczgwhjthfhztzrlpgzjhcfrjpjvtjptptbvflftjtcfhmbwlhlbhvnjnbfmwjrgbvvhmdlncdgncgfjcnnpdljfcjsmsfscqpwsgcmlhhqmldsnjfrrqpghwncmgwgnjsdtvbhrbbnmpqjrrctqqnqzztmbqmdsgdvmmlwmbvprllzgntnmttrlzrttmjjlrwpwmtfznmwnsjmjhjdnsppfhcrjpzhjqzdtdbsjshfzzvrwvjbjbgtsfpgggbdztczwlhpmthfjdgsbrvlwmlrvgdrpjzccwmgpcnqqzmqdjqmwsrzwsmtmdjdhmjrwfwnzlmfnqtcgtslwtlnwhvmqntmglhntnsjlnmzfvfdztcfwmpchsrsdmqvqcwljzrmmssjvbmvvnmqlbsdwnrbmqctdtmfzlgfzpmjcnftgftvjpfbwwmzfdrrwjwcfwfcfmzbbnppgjrmbcvmvnjpdrzmvndvddtvshlnjjwgtsvnwtwnhcbfpnthpjlrhgrqccdgppjvdqjwqrfrrgnvhfwvjhnwhntnpmghphrtgqhwtbrqhqljfdjbgnlgmqqgfcqpqfhcpgspdbvlbfjvlrgmtjztwdzlrhqwwtcpdvsqgssjbjjgqlwbcctzzqvvmdzpfrmspmqhtzwgcfsslpnhpjfwqrrfbwbndrvhnnsjnlvlvqdsgwzjsrprhgtvsfbhbcpljdczbtdwzcnhzntrwcrjctmhtjfdlthznzmqblppzcqgpjhlzjrmcvpptfjjzltdhmvwphwlccscwrwfcqpqwwrzcmnltzdcfvtjrcvsqwtchrmdfzjmzjfhppjzbhglwqggzqqnspfmzrfwrqdqdrsdbsdhcgdqrrnjlwrqhfhpzjhrvjndqphndnnnbwhrjvqrrbvlhhbljjcwmfpvnhcszfshlsnczgtcfhjslbhzczdqdmdnvqdzhbmbpcnbntwgllfscrcwhfrgtfvftmwhbgfhjzjrbvvwc
+
diff --git a/day_6/lexer.l b/day_6/lexer.l
new file mode 100644
index 0000000..0bc05d7
--- /dev/null
+++ b/day_6/lexer.l
@@ -0,0 +1,41 @@
+%{
+#include <stdio.h>
+#include <stdlib.h>
+#include "parser.h"
+%}
+
+%option warn nodefault
+
+/* makes the scanner terminate after reaching <<EOF>> instead of assuming a new input was provided */
+%option noyywrap
+/* disable some unused functionality, add scanner tracking */
+%option nounput noinput batch debug
+
+/* gimme a reentrant parser (overkill but more pure) */
+%option reentrant
+
+%option bison-bridge
+
+
+NL                  [\n]
+CHAR                [a-z]
+
+
+%%
+
+{NL}                {
+                        return NEWLINE;
+                    }
+{CHAR}              {
+                        yylval->cval = yytext[0];
+                        return CHAR;
+                    }
+<<EOF>>             {
+                        return END_OF_FILE;
+                    }
+.                   {
+                        printf("[error] Encountered unexpected token %s\n", yytext);
+                        return 0;
+                    }
+
+%%
diff --git a/day_6/parser.y b/day_6/parser.y
new file mode 100644
index 0000000..6a9be01
--- /dev/null
+++ b/day_6/parser.y
@@ -0,0 +1,139 @@
+/* Require bison minimal version */
+%require "3.2"
+
+/* write out a header file containing the token defines */
+%header
+
+// Code for the header file generated by bison
+%code requires {
+    #include <stdbool.h>
+
+    typedef void* yyscan_t;
+
+    struct parser_state {
+        // for storing the most recently read 4 chars
+        char buffer[4];
+        // location tracker
+        unsigned long loc;
+        bool found_start;
+
+        bool check_matrix[26];
+    };
+}
+
+// Code for the c file
+%{
+    #include <stdio.h>
+    #include <stdlib.h>
+    #include <stdbool.h>
+    #include "parser.h"
+    #include "lexer.h"
+
+    void yyerror(struct parser_state* state, yyscan_t scanner, const char* msg) {
+        (void)state;
+        (void)scanner;
+        fprintf(stderr, "\033[93mSyntax Error: %s\033[0m\n", msg);
+    }
+%}
+
+// define a reentrant parser
+%define api.pure
+
+// enable parser tracing and detailed error messages (plus Lookahead Correction)
+%define parse.trace
+%define parse.error detailed
+%define parse.lac full
+
+%lex-param   { yyscan_t scanner }
+%parse-param { struct parser_state* state }
+%parse-param { yyscan_t scanner }
+
+%union {
+    char cval;
+}
+
+
+%start input
+%token NEWLINE
+%token <cval> CHAR
+%term END_OF_FILE
+
+%%
+
+input
+    : msg blanks END_OF_FILE   { return 0; }
+    ;
+
+blanks
+    : NEWLINE blanks
+    | NEWLINE
+    ;
+
+msg
+    : bit msg
+    | bit
+    ;
+
+bit
+    : CHAR              {
+                            if (!state->found_start) {
+                                state->loc++;
+
+                                // left shift existing data
+                                for (int i = 0; i < 3; i++) {
+                                    state->buffer[i] = state->buffer[i+1];
+                                }
+
+                                state->buffer[3] = $1;
+
+                                // check whether condition is met
+                                // offset array indices by the base -- 'a'
+                                char offset = 'a';
+
+                                state->found_start = true;
+                                for (int i = 0; i < 4; i++) {
+                                    if (!state->check_matrix[state->buffer[i] - offset]) {
+                                        state->check_matrix[state->buffer[i] - offset] = true;
+                                    } else {
+                                        state->found_start = false;
+                                        break;
+                                    }
+                                }
+
+                                // housekeeping; reset the fields to false
+                                for (int i = 0; i < 4; i++) {
+                                    state->check_matrix[state->buffer[i] - offset] = false;
+                                }
+                            }
+                        }
+    ;
+
+
+%%
+
+int main(void) {
+    struct parser_state parser_state;
+    for (int i = 0; i < 4; i++) {
+        parser_state.buffer[i] = 'a';
+    }
+    parser_state.loc = 0;
+    parser_state.found_start = false;
+
+    yyscan_t scanner;
+    if (yylex_init(&scanner)) {
+        fprintf(stderr, "\033[91m[error] Could not initialize lexer\033[0m\n");
+        return EXIT_FAILURE;
+    }
+
+    if (yyparse(&parser_state, scanner)) {
+		// TODO: Error handling https://www.gnu.org/software/bison/manual/html_node/Parser-Function.html
+		// error during parse occured
+		return EXIT_FAILURE;
+	}
+
+    yylex_destroy(scanner);
+
+    printf("First start-of-packet marker detected at: %lu\n", parser_state.loc);
+
+    return 0;
+}