diff --git a/common/filterpaper.h b/common/filterpaper.h
new file mode 100644
index 0000000..9598dc6
--- /dev/null
+++ b/common/filterpaper.h
@@ -0,0 +1,87 @@
+// Copyright @filterpaper
+// Copyright Christoph Cullmann
+// SPDX-License-Identifier: GPL-2.0+
+
+// Basic keycode filter for tap-hold keys
+#define GET_TAP_KEYCODE(k) ((k) & 0xff)
+
+// Tap-hold decision helper macros
+#define IS_LAYER_TAP(k)     (IS_QK_LAYER_TAP(k) && QK_LAYER_TAP_GET_LAYER(k))
+#define IS_SHORTCUT(k)      (IS_QK_LAYER_TAP(k) && !QK_LAYER_TAP_GET_LAYER(k))
+#define IS_MOD_TAP_SHIFT(k) (IS_QK_MOD_TAP(k) && (k) & (QK_LSFT))
+#define IS_MOD_TAP_CAG(k)   (IS_QK_MOD_TAP(k) && (k) & (QK_LCTL|QK_LALT|QK_LGUI))
+
+#define IS_HOMEROW(r)          (r->event.key.row == 1 || r->event.key.row == 5)
+#define IS_HOMEROW_SHIFT(k, r) (IS_HOMEROW(r) && IS_MOD_TAP_SHIFT(k))
+#define IS_HOMEROW_CAG(k, r)   (IS_HOMEROW(r) && IS_MOD_TAP_CAG(k))
+
+#define IS_TYPING(k) ((uint8_t)(k) <= KC_Z && last_input_activity_elapsed() < QUICK_TAP_TERM)
+
+#define IS_UNILATERAL(r, n) ( \
+    (r->event.key.row == 1 && 0 <= n.event.key.row && n.event.key.row <= 2) || \
+    (r->event.key.row == 5 && 4 <= n.event.key.row && n.event.key.row <= 6) )
+
+#define IS_BILATERAL(r, n) ( \
+    (r->event.key.row == 1 && 4 <= n.event.key.row && n.event.key.row <= 6) || \
+    (r->event.key.row == 5 && 0 <= n.event.key.row && n.event.key.row <= 2) )
+
+static uint_fast16_t inter_keycode;
+static keyrecord_t   inter_record;
+
+bool pre_process_record_user(uint16_t keycode, keyrecord_t *record) {
+    static bool     is_pressed[UINT8_MAX];
+    const  uint16_t tap_keycode = GET_TAP_KEYCODE(keycode);
+
+    if (record->event.pressed) {
+        // Press the tap keycode if the tap-hold key follows the previous key swiftly
+        if ((IS_HOMEROW_CAG(keycode, record) || IS_SHORTCUT(keycode)) && IS_TYPING(inter_keycode)) {
+            is_pressed[tap_keycode] = true;
+            record->keycode = tap_keycode;
+        }
+        // Cache incoming input for in-progress and subsequent tap-hold decisions
+        inter_keycode = keycode;
+        inter_record  = *record;
+    }
+
+    // Release the pressed tap keycode
+    else if (is_pressed[tap_keycode]) {
+        is_pressed[tap_keycode] = false;
+        record->keycode = tap_keycode;
+    }
+
+    return true;
+}
+
+bool get_hold_on_other_key_press(uint16_t keycode, keyrecord_t *record) {
+    // Tap the mod-tap key with an overlapping non-Shift key on the same hand
+    // or the shortcut key with any overlapping key
+    if ((IS_UNILATERAL(record, inter_record) && !IS_MOD_TAP_SHIFT(inter_keycode)) || IS_SHORTCUT(keycode)) {
+        record->tap.count++;
+        process_record(record);
+        return false;
+    }
+
+    // Activate layer hold with another key press
+    else return IS_LAYER_TAP(keycode);
+}
+
+bool get_permissive_hold(uint16_t keycode, keyrecord_t *record) {
+    // Enable Shift with a nested key press
+    return IS_MOD_TAP_SHIFT(keycode);
+}
+
+uint16_t get_tapping_term(uint16_t keycode, keyrecord_t *record) {
+    // Shorten interval for Shift
+    return IS_HOMEROW_SHIFT(keycode, record) ? SHIFT_TAP_TERM : TAPPING_TERM;
+}
+
+// Simplify unused magic config functions
+#ifndef MAGIC_ENABLE
+uint8_t mod_config(uint8_t mod) { return mod; }
+uint16_t keycode_config(uint16_t keycode) { return keycode; }
+#endif
+
+// Reduce marix scanning delay
+#ifndef DIRECT_PINS
+void matrix_io_delay(void) { __asm__ volatile("nop\nnop\nnop\n"); }
+#endif