quantum/debounce: rename debouncing algorithms (#9564)

* quantum/debounce: rename debouncing algorithms according to Issue 8763

This is the second attempt at implementation, with no ts_ and cy_ prefixes, since those will be implemented with macros.

* Debouncing documentation: Refactor, add some generic info, and merge into a single document

7 files changed, 139 insertions, 48 deletions

diff --git a/common_features.mk b/common_features.mk
index 1f110d0819..ed6908f4b8 100644
--- a/common_features.mk
+++ b/common_features.mk
@@ -397,9 +397,20 @@ ifneq ($(strip $(CUSTOM_MATRIX)), yes)
     endif
 endif
 
+# Support for translating old names to new names:
+ifeq ($(strip $(DEBOUNCE_TYPE)),sym_g)
+    DEBOUNCE_TYPE:=sym_defer_g
+else ifeq ($(strip $(DEBOUNCE_TYPE)),eager_pk)
+    DEBOUNCE_TYPE:=sym_eager_pk
+else ifeq ($(strip $(DEBOUNCE_TYPE)),sym_pk)
+    DEBOUNCE_TYPE:=sym_defer_pk
+else ifeq ($(strip $(DEBOUNCE_TYPE)),eager_pr)
+    DEBOUNCE_TYPE:=sym_eager_pr
+endif
+
 DEBOUNCE_DIR:= $(QUANTUM_DIR)/debounce
 # Debounce Modules. Set DEBOUNCE_TYPE=custom if including one manually.
-DEBOUNCE_TYPE?= sym_g
+DEBOUNCE_TYPE?= sym_defer_g
 ifneq ($(strip $(DEBOUNCE_TYPE)), custom)
     QUANTUM_SRC += $(DEBOUNCE_DIR)/$(strip $(DEBOUNCE_TYPE)).c
 endif
diff --git a/docs/feature_debounce_type.md b/docs/feature_debounce_type.md
index 65b4ea1e53..83ebafe60e 100644
--- a/docs/feature_debounce_type.md
+++ b/docs/feature_debounce_type.md
@@ -1,43 +1,151 @@
-# Debounce algorithm
+# Contact bounce / contact chatter
 
-QMK supports multiple debounce algorithms through its debounce API.
+Mechanical switches often don't have a clean single transition between pressed and unpressed states.
+
+In an ideal world, when you press a switch, you would expect the digital pin to see something like this:
+(X axis showing time
+```
+voltage                   +----------------------
+ ^                        |
+ |                        |
+ |      ------------------+
+          ----> time
+```
+
+However in the real world you will actually see contact bounce, which will look like multiple 1->0 and 0->1 transitions,
+until the value finally settles.
+```
+                  +-+ +--+ +-------------
+                  | | |  | |
+                  | | |  | |
++-----------------+ +-+  +-+
+```
+The time it takes for the switch to settle might vary with switch type, age, and even pressing technique.
+
+If the device chooses not to mitigate contact bounce, then often actions that happen when the switch is pressed are repeated
+multiple times.
+
+There are many ways to handle contact bounce ("Debouncing"). Some include employing additional hardware, for example an RC filter,
+while there are various ways to do debouncing in software too, often called debounce algorithms. This page discusses software
+debouncing methods available in QMK.
+
+While technically not considered contact bounce/contact chatter, some switch technologies are susceptible to noise, meaning,
+while the key is not changing state, sometimes short random 0->1 or 1->0 transitions might be read by the digital circuit, for example:
+```
+                  +-+
+                  | |
+                  | |
++-----------------+ +--------------------
+```
+
+Many debounce methods (but not all) will also make the device resistant to noise. If you are working with a technology that is
+susceptible to noise, you must choose a debounce method that will also mitigate noise for you.
+
+## Types of debounce algorithms
 
-The logic for which debounce method called is below. It checks various defines that you have set in rules.mk
+1) Unit of time: Timestamp (milliseconds) vs Cycles (scans)
+   * Debounce algorithms often have a 'debounce time' parameter, that specifies the maximum settling time of the switch contacts.
+     This time might be measured in various units:
+     * Cycles-based debouncing waits n cycles (scans), decreasing count by one each matrix_scan
+     * Timestamp-based debouncing stores the millisecond timestamp a change occurred, and does substraction to figure out time elapsed.
+   * Timestamp-based debouncing is usually superior, especially in the case of noise-resistant devices because settling times of physical
+     switches is specified in units of time, and should not depend on the matrix scan-rate of the keyboard.
+   * Cycles-based debouncing is sometimes considered inferior, because the settling time that it is able to compensate for depends on the
+     performance of the matrix scanning code. If you use cycles-based debouncing, and you significantly improve the performance of your scanning
+     code, you might end up with less effective debouncing. A situation in which cycles-based debouncing might be preferable is when
+     noise is present, and the scanning algorithm is slow, or variable speed. Even if your debounce algorithm is fundamentally noise-resistant,
+     if the scanning is slow, and you are using a timestamp-based algorithm, you might end up making a debouncing decision based on only two
+     sampled values, which will limit the noise-resistance of the algorithm.
+   * Currently all built-in debounce algorithms support timestamp-based debouncing only. In the future we might
+     implement cycles-based debouncing, and it will be selectable via a ```config.h``` macro.
+
+2) Symmetric vs Asymmetric
+   * Symmetric - apply the same debouncing algorithm, to both key-up and key-down events.
+     * Recommended naming convention: ```sym_*```
+   * Asymmetric - apply different debouncing algorithms to key-down and key-up events. E.g. Eager key-down, Defer key-up.
+     * Recommended naming convention: ```asym_*``` followed by details of the type of algorithm in use, in order, for key-down and then key-up
+
+3) Eager vs Defer
+   * Eager - any key change is reported immediately. All further inputs for DEBOUNCE ms are ignored.
+     * Eager algorithms are not noise-resistant.
+     * Recommended naming conventions:
+        * ```sym_eager_*```
+        * ```asym_eager_*_*```: key-down is using eager algorithm
+        * ```asym_*_eager_*```: key-up is using eager algorithm
+   * Defer - wait for no changes for DEBOUNCE ms before reporting change.
+     * Defer algorithms are noise-resistant
+     * Recommended naming conventions:
+        * ```sym_defer_*```
+        * ```asym_defer_*_*```: key-down is using eager algorithm
+        * ```asym_*_defer_*```: key-up is using eager algorithm
+
+4) Global vs Per-Key vs Per-Row
+   * Global - one timer for all keys. Any key change state affects global timer
+     * Recommended naming convention: ```*_g```
+   * Per-key - one timer per key
+     * Recommended naming convention: ```*_pk```
+   * Per-row - one timer per row
+     * Recommended naming convention: ```*_pr```
+   * Per-key and per-row algorithms consume more resources (in terms of performance,
+     and ram usage), but fast typists might prefer them over global.
+
+## Debounce algorithms supported by QMK
+
+QMK supports multiple debounce algorithms through its debounce API.
+The logic for which debounce method called is below. It checks various defines that you have set in ```rules.mk```
 
 ```
 DEBOUNCE_DIR:= $(QUANTUM_DIR)/debounce
-DEBOUNCE_TYPE?= sym_g
+DEBOUNCE_TYPE?= sym_defer_g
 ifneq ($(strip $(DEBOUNCE_TYPE)), custom)
     QUANTUM_SRC += $(DEBOUNCE_DIR)/$(strip $(DEBOUNCE_TYPE)).c
 endif
 ```
 
-# Debounce selection
+### Debounce selection
 
 | DEBOUNCE_TYPE    | Description                                          | What else is needed           |
 | -------------    | ---------------------------------------------------  | ----------------------------- |
-| Not defined      | Use the default algorithm, currently sym_g           | Nothing                       |
+| Not defined      | Use the default algorithm, currently sym_defer_g     | Nothing                       |
 | custom           | Use your own debounce code                           | ```SRC += debounce.c``` add your own debounce.c and implement necessary functions |
-| anything_else    | Use another algorithm from quantum/debounce/*        | Nothing                       |
+| Anything Else    | Use another algorithm from quantum/debounce/*        | Nothing                       |
 
 **Regarding split keyboards**:
 The debounce code is compatible with split keyboards.
 
-# Use your own debouncing code
-* Set ```DEBOUNCE_TYPE = custom```.
-* Add ```SRC += debounce.c```
+### Selecting an included debouncing method
+Keyboards may select one of the already implemented debounce methods, by adding to ```rules.mk``` the following line:
+```
+DEBOUNCE_TYPE = <name of algorithm>
+```
+Where name of algorithm is one of:
+* ```sym_defer_g``` - debouncing per keyboard. On any state change, a global timer is set. When ```DEBOUNCE``` milliseconds of no changes has occurred, all input changes are pushed.
+  * This is the current default algorithm. This is the highest performance algorithm with lowest memory usage, and it's also noise-resistant.
+* ```sym_eager_pr``` - debouncing per row. On any state change, response is immediate, followed by locking the row ```DEBOUNCE``` milliseconds of no further input for that row. 
+For use in keyboards where refreshing ```NUM_KEYS``` 8-bit counters is computationally expensive / low scan rate, and fingers usually only hit one row at a time. This could be
+appropriate for the ErgoDox models; the matrix is rotated 90°, and hence its "rows" are really columns, and each finger only hits a single "row" at a time in normal use.
+* ```sym_eager_pk``` - debouncing per key. On any state change, response is immediate, followed by ```DEBOUNCE``` milliseconds of no further input for that key
+* ```sym_defer_pk``` - debouncing per key. On any state change, a per-key timer is set. When ```DEBOUNCE``` milliseconds of no changes have occurred on that key, the key status change is pushed.
+
+### A couple algorithms that could be implemented in the future:
+* ```sym_defer_pr```
+* ```sym_eager_g```
+* ```asym_eager_defer_pk```
+
+### Use your own debouncing code
+You have the option to implement you own debouncing algorithm. To do this:
+* Set ```DEBOUNCE_TYPE = custom``` in ```rules.mk```.
+* Add ```SRC += debounce.c``` in ```rules.mk```
 * Add your own ```debounce.c```. Look at current implementations in ```quantum/debounce``` for examples.
 * Debouncing occurs after every raw matrix scan.
 * Use num_rows rather than MATRIX_ROWS, so that split keyboards are supported correctly.
+* If the algorithm might be applicable to other keyboards, please consider adding it to ```quantum/debounce```
 
-# Changing between included debouncing methods
-You can either use your own code, by including your own debounce.c, or switch to another included one.
-Included debounce methods are:
-* eager_pr - debouncing per row. On any state change, response is immediate, followed by locking the row ```DEBOUNCE``` milliseconds of no further input for that row. 
-For use in keyboards where refreshing ```NUM_KEYS``` 8-bit counters is computationally expensive / low scan rate, and fingers usually only hit one row at a time. This could be
-appropriate for the ErgoDox models; the matrix is rotated 90°, and hence its "rows" are really columns, and each finger only hits a single "row" at a time in normal use.
-* eager_pk - debouncing per key. On any state change, response is immediate, followed by ```DEBOUNCE``` milliseconds of no further input for that key
-* sym_g - debouncing per keyboard. On any state change, a global timer is set. When ```DEBOUNCE``` milliseconds of no changes has occured, all input changes are pushed.
-* sym_pk - debouncing per key. On any state change, a per-key timer is set. When ```DEBOUNCE``` milliseconds of no changes have occured on that key, the key status change is pushed.
+### Old names
+The following old names for existing algorithms will continue to be supported, however it is recommended to use the new names instead.
 
+* sym_g - old name for sym_defer_g
+* eager_pk - old name for sym_eager_pk
+* sym_pk - old name for sym_defer_pk
+* eager_pr - old name for sym_eager_pr
 
diff --git a/quantum/debounce/readme.md b/quantum/debounce/readme.md
deleted file mode 100644
index f77f78c764..0000000000
--- a/quantum/debounce/readme.md
+++ /dev/null
@@ -1,28 +0,0 @@
-Debounce algorithms belong in this folder.
-Here are a few ideas
-
-1) Global vs Per-Key vs Per-Row
- * Global - one timer for all keys. Any key change state affects global timer
- * Per key - one timer per key
- * Per row - one timer per row
-
-2) Eager vs symmetric vs asymmetric
- * Eager - any key change is reported immediately. All further inputs for DEBOUNCE ms are ignored.
- * Symmetric - wait for no changes for DEBOUNCE ms before reporting change
- * Asymmetric - wait for different times depending on key-down/key-up. E.g. Eager key-down, DEBOUNCE ms key up.
-
-3) Timestamp vs cycles
- * old old old code waits n cycles, decreasing count by one each matrix_scan
- * newer code stores the millisecond the change occurred, and does subraction to figure out time elapsed.
- * Timestamps are superior, i don't think cycles will ever be used again once upgraded.
-
-The default algorithm is symmetric and global.
-Here are a few that could be implemented:
-
-sym_g.c
-sym_pk.c
-sym_pr.c
-sym_pr_cycles.c 
-eager_g.c
-eager_pk.c
-eager_pr.c //could be used in ergo-dox!
diff --git a/quantum/debounce/sym_g.c b/quantum/debounce/sym_defer_g.c
index 3ed9055d2a..3ed9055d2a 100644
--- a/quantum/debounce/sym_g.c
+++ b/quantum/debounce/sym_defer_g.c
diff --git a/quantum/debounce/sym_pk.c b/quantum/debounce/sym_defer_pk.c
index f404cf9c44..f404cf9c44 100644
--- a/quantum/debounce/sym_pk.c
+++ b/quantum/debounce/sym_defer_pk.c
diff --git a/quantum/debounce/eager_pk.c b/quantum/debounce/sym_eager_pk.c
index 93a40ad441..93a40ad441 100644
--- a/quantum/debounce/eager_pk.c
+++ b/quantum/debounce/sym_eager_pk.c
diff --git a/quantum/debounce/eager_pr.c b/quantum/debounce/sym_eager_pr.c
index d12931fddb..d12931fddb 100644
--- a/quantum/debounce/eager_pr.c
+++ b/quantum/debounce/sym_eager_pr.c