diff --git a/sudoku.c b/sudoku.c
new file mode 100644
index 0000000..5c079b7
--- /dev/null
+++ b/sudoku.c
@@ -0,0 +1,327 @@
+#include <stdio.h>
+#include <stdbool.h>
+
+#define UNDEFINED -1
+#define MULTIPLE -2
+#define FIELD_UNDEFINED UNDEFINED
+#define FIELD_MULTIPLE MULTIPLE
+
+#define VALUE_UNDEFINED UNDEFINED
+#define VALUE_MULTIPLE MULTIPLE
+#define VALUE_ALREADY_SET -3
+
+
+// https://en.wikipedia.org/wiki/Sudoku#/media/File:Sudoku_Puzzle_by_L2G-20050714_standardized_layout.svg
+/*
+int sudoku[9][9] = {
+    {5,3,0,6,0,0,0,9,8},
+    {0,7,0,1,9,5,0,0,0},
+    {0,0,0,0,0,0,0,6,0},
+    {8,0,0,4,0,0,7,0,0},
+    {0,6,0,8,0,3,0,2,0},
+    {0,0,3,0,0,1,0,0,6},
+    {0,6,0,0,0,0,0,0,0},
+    {0,0,0,4,1,9,0,8,0},
+    {2,8,0,0,0,5,0,7,9}
+};
+*/
+
+// https://www.free-sudoku.com/sudoku.php?dchoix=evil
+int sudoku[9][9] = {
+    {VALUE_UNDEFINED,1,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED},
+    {3,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,8,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED},
+    {VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,4,VALUE_UNDEFINED,VALUE_UNDEFINED,7,VALUE_UNDEFINED},
+    {4,VALUE_UNDEFINED,5,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,8,VALUE_UNDEFINED,VALUE_UNDEFINED},
+    {VALUE_UNDEFINED,7,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,2,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED},
+    {VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,1,VALUE_UNDEFINED,VALUE_UNDEFINED,3,VALUE_UNDEFINED,VALUE_UNDEFINED},
+    {VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,7,VALUE_UNDEFINED,VALUE_UNDEFINED},
+    {1,VALUE_UNDEFINED,VALUE_UNDEFINED,6,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED},
+    {5,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,8,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED,VALUE_UNDEFINED}
+};
+
+void readQ(int num) {
+    printf("Enter every number of Q%d from left top to right bottom. Use `0` if on no initial value:\n", num + 1);
+    for (int i = 0; i < 9; i++) {
+        scanf("%d", &sudoku[num][i]);
+    }
+}
+
+int calculateQuadrat(int row, int col) {
+    return (row / 3) * 3 + col / 3;
+}
+
+int calculateField(int row, int col) {
+    return (row % 3) * 3 + col % 3;
+}
+
+int calculateRow(int q, int f) {
+    return (q / 3) * 3 + (f / 3);
+}
+
+int calculateCol(int q, int f) {
+    return (q % 3) * 3 + (f % 3);
+}
+
+void printDumpSudoku() {
+    for (int row = 0; row < 9; row++) {
+        if (row % 3 == 0) {
+            printf("+-------+-------+-------+\n");
+        }
+        for (int col = 0; col < 9; col++) {
+            if (col % 3 == 0) {
+                if (col != 0) {
+                    printf(" ");
+                }
+                printf("|");
+            }
+
+            int q = calculateQuadrat(row, col);
+            int f = calculateField(row, col);
+            if (sudoku[q][f] > VALUE_UNDEFINED) {
+                printf(" %d", sudoku[q][f]);
+            } else {
+                printf(" -");
+            }
+        }
+        printf(" |\n");
+    }
+    printf("+-------+-------+-------+\n");
+}
+
+void printDumpQuadrat(int q[]) {
+    for (int row = 0; row < 3; row++) {
+        if (row % 3 == 0) {
+            printf("+-------+\n");
+        }
+        for (int col = 0; col < 3; col++) {
+            if (col % 3 == 0) {
+                if (col != 0) {
+                    printf(" ");
+                }
+                printf("|");
+            }
+
+            int f = calculateField(row, col);
+            printf(" %d", q[f]);
+        }
+        printf(" |\n");
+    }
+    printf("+-------+\n");
+}
+
+// checks if this value is present anywhere in the row
+bool isInRow(int row, int val) {
+    for (int col = 0; col < 9; col++) {
+        int q = calculateQuadrat(row, col);
+        int f = calculateField(row, col);
+
+        if (sudoku[q][f] == val) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+// checks if this value is present anywhere in the col
+bool isInCol(int col, int val) {
+    for (int row = 0; row < 9; row++) {
+        int q = calculateQuadrat(row, col);
+        int f = calculateField(row, col);
+
+        if (sudoku[q][f] == val) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+// special version of isInRow where it checks if the value
+// has to be in the row in special quadrat except the one
+// we are currently looking at
+bool isOnlyInOneRowQuadrat(int notThisQuadrat, int row, int val) {
+    int initQ = calculateQuadrat(row, 0);
+
+    for (int q = initQ; q < initQ + 3; q++) {
+        if (q == notThisQuadrat) {
+            continue;
+        }
+
+        bool skipQuadrat = false;
+        bool possibleRowFound = false;
+        for (int f = 0; f < 9 && !skipQuadrat; f++) {
+            int r = calculateRow(q, f);
+            int c = calculateCol(q, f);
+
+            if (sudoku[q][f] == val) {
+                if (r == row) {
+                    return true;
+                } else {
+                    skipQuadrat = true;
+                    continue;
+                }
+            } else if (sudoku[q][f] <= VALUE_UNDEFINED && !isInRow(r, val) && !isInCol(c, val)) {
+                if (r != row) {
+                    return false;
+                } else {
+                    possibleRowFound = true;
+                }
+            } else {
+                // field is either already set to another value or value is present in current row or col
+                // either way, this field is not a possible option for the value
+            }
+        }
+
+        if (!skipQuadrat && possibleRowFound) {
+            // we found a possible field in row which may be filled with val
+            // and we didn't hit any abort condition for current quadrat
+            return true;
+        }
+    }
+
+    // we didn't found a possible field in row and we didn't hit any
+    // abort condition. This is may happen if val is already set in bot
+    // quadrats but not in searched row
+    return false;
+}
+
+// special version of isInCol where it checks if the value
+// has to be in the col in special quadrat except the one
+// we are currently looking at
+bool isOnlyInOneColQuadrat(int notThisQuadrat, int col, int val) {
+    int initQ = calculateQuadrat(0, col);
+
+    for (int q = initQ; q <= initQ + 6; q += 3) {
+        if (q == notThisQuadrat) {
+            continue;
+        }
+
+        bool skipQuadrat = false;
+        bool possibleColFound = false;
+        for (int f = 0; f < 9 && !skipQuadrat; f++) {
+            int r = calculateRow(q, f);
+            int c = calculateCol(q, f);
+
+            if (sudoku[q][f] == val) {
+                if (c == col) {
+                    return true;
+                } else {
+                    skipQuadrat = true;
+                    continue;
+                }
+            } else if (sudoku[q][f] <= VALUE_UNDEFINED && !isInRow(r, val) && !isInCol(c, val)) {
+                if (c != col) {
+                    return false;
+                } else {
+                    possibleColFound = true;
+                }
+            } else {
+                // field is either already set to another value or value is present in current row or col
+                // either way, this field is not a possible option for the value
+            }
+        }
+
+        if (!skipQuadrat && possibleColFound) {
+            // we found a possible field in row which may be filled with val
+            // and we didn't hit any abort condition for current quadrat
+            return true;
+        }
+    }
+
+    // we didn't found a possible field in row and we didn't hit any
+    // abort condition. This is may happen if val is already set in bot
+    // quadrats but not in searched row
+    return false;
+}
+
+// checks if this value is present anywhere in the quadrat
+bool isInQuadrat(int q, int val) {
+    for (int f = 0; f < 9; f++) {
+        if (sudoku[q][f] == val) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+void setValue(int q, int f, int val) {
+    sudoku[q][f] = val;
+    printf("=> setting %d in Q %d; F %d\n", val, q, f);
+    printDumpSudoku();
+}
+
+int main() {
+    printDumpSudoku();
+    bool somethingChanged;
+    do {
+        somethingChanged = false;
+        for (int q = 0; q < 9; q++) {
+            int possbileVal[9] = {
+                VALUE_UNDEFINED, VALUE_UNDEFINED, VALUE_UNDEFINED,
+                VALUE_UNDEFINED, VALUE_UNDEFINED, VALUE_UNDEFINED,
+                VALUE_UNDEFINED, VALUE_UNDEFINED, VALUE_UNDEFINED
+            };
+
+            for (int val = 1; val <= 9; val++) {
+                if (isInQuadrat(q, val)) {
+                    continue;
+                }
+
+                int possibleField = FIELD_UNDEFINED;
+                for (int f = 0; f < 9; f++) {
+                    int row = calculateRow(q, f);
+                    int col = calculateCol(q, f);
+
+                    // look for val in every row and col
+                    // cache evaluation of isInRow and isInCol
+                    if (sudoku[q][f] <= VALUE_UNDEFINED
+                            && !(isInRow(row, val) || isOnlyInOneRowQuadrat(q, row, val))
+                            && !(isInCol(col, val) || isOnlyInOneColQuadrat(q, col, val))) {
+                            // && !isInRow(row, val)
+                            // && !isInCol(col, val)) {
+                        if (possibleField == FIELD_UNDEFINED) {
+                            possibleField = f;
+                        } else {
+                            possibleField = FIELD_MULTIPLE;
+                        }
+
+                        if (possbileVal[f] == VALUE_UNDEFINED) {
+                            possbileVal[f] = val;
+                        } else if (possbileVal[f] > VALUE_UNDEFINED) {
+                            possbileVal[f] = VALUE_MULTIPLE;
+                        } else {
+                            // already set to multiple or any other meta information
+                        }
+                    }
+                }
+
+                // found possible field? set val
+                if (possibleField > FIELD_UNDEFINED) {
+                    setValue(q, possibleField, val);
+                    somethingChanged = true;
+                }
+            }
+
+            // check if only one field is left undefined -> hast to be the one special value
+            int undefinedFound = VALUE_UNDEFINED;
+            for (int f = 0; f < 9; f++) {
+                // keep in mind that sudoku array might have changed since evaluation
+                // -> ignore already set values
+                if (sudoku[q][f] > VALUE_UNDEFINED) {
+                    continue;
+                }
+
+                if (possbileVal[f] > VALUE_UNDEFINED) {
+                    setValue(q, f, possbileVal[f]);
+                    somethingChanged = true;
+                } else {
+                    // set to undefined, multiple or any other meta information
+                }
+            }
+        }
+    } while (somethingChanged);
+
+    // next step: muss in Feld X sein, weil alle anderen Felder irgendwie anders belegt sein müssen (aber noch nicht sind)
+}
\ No newline at end of file
diff --git a/test.c b/test.c
new file mode 100644
index 0000000..f792609
--- /dev/null
+++ b/test.c
@@ -0,0 +1,15 @@
+#include <stdio.h>
+
+int main() {
+    for (int row = 0; row < 9; row++) {
+        for (int col = 0; col < 9; col++) {
+            printf("row: %d col: %d Q: %d F: %d\n", row, col, (row / 3) * 3 + col / 3, (row % 3) * 3 + col % 3);
+        }
+    }
+
+    for (int q = 0; q < 9; q++) {
+        for (int f = 0; f < 9; f++) {
+            printf("Q: %d F: %d row: %d col: %d\n", q, f, (q / 3) * 3 + (f / 3), (q % 3) * 3 + (f % 3));
+        }
+    }
+}
\ No newline at end of file