nearing the deadline, more implementation details

main.typ

@@ -623,8 +623,8 @@ Let's now recap the main formal algorithm for computing the Kauffman polynomial.
       $K$, $S_i K$, $E_i K$, $e_i K$,
       [ #set text(size: 9pt); _original_],
       [ #set text(size: 9pt); _switch_],
-      [ #set text(size: 9pt); _splice_],
-      [ #set text(size: 9pt); _splice_],
+      [ #set text(size: 9pt); _h-splice_],
+      [ #set text(size: 9pt); _v-splice_],
     ),
   )
 ]
@@ -634,9 +634,9 @@ Let now $K$ be an oriented link with $n$ components so $K = K_1 union ... union
 #definition[
   Let $K$ abd $lambda = (lambda_n, ..., lambda_0)$ a sequence of indices of crossing of $K$ and let $i$ be an index of one of the crossings, let's define the following actions
 
-  - $A_i^lambda colon.eq E_i S_(lambda_i) ... S_(lambda_0)$
+  - $A_i^lambda K colon.eq E_i S_(lambda_i) dots.c space S_(lambda_0) K$
 
-  - $B_i^lambda colon.eq e_i S_(lambda_i) ... S_(lambda_0)$
+  - $B_i^lambda K colon.eq e_i S_(lambda_i) dots.c space S_(lambda_0) K$
 
   - Then let $lambda$ be a sequence of indices that bring $K$ to $hat(K)$ so that $hat(K)(lambda) colon.eq S_(lambda_n) dots.c space S_(lambda_0) K$ and define
 
@@ -687,6 +687,176 @@ Let now $K$ be an oriented link with $n$ components so $K = K_1 union ... union
       $
     ]
 
+
+== Python Implementation
+
+The approach has been a mix of bottom-up and top-down. First we defined a couple of classed `SignedGaussCode` and `PDCode` to work with these codes and easily convert between each other.
+
+This initial implementation uses `SignedGaussCodes` as they are easier to work with when working with crossing switches and splices but with some modifications the code could be adapted to work directly on `PDCode` provided of some efficient implementations of `splice_h` and `splice_v` methods.
+
+#pagebreak()
+
+=== Signed Gauss Codes
+
+We are now going to walk thorough the class that lets use work nicely with *Signed Gauss Codes*. The the classes we are going to use are all _frozen data-classes_ to ensure immutability and enforce a more functional programming style.
+
+#show raw.where(block: true): body => {
+  set text(size: 7pt)
+  set align(center)
+
+  body
+}
+
+```python
+Sign = typing.Literal[+1, -1]
+
+@dataclass(frozen=True)
+class SignedGaussCodeCrossing:
+    id: int
+    over_under: Sign
+    handedness: Sign
+
+    def is_over(self) -> bool: ...
+    def is_under(self) -> bool: ...
+    def is_left(self) -> bool: ...
+    def is_right(self) -> bool: ...
+    def opposite(self) -> SignedGaussCodeCrossing: ...
+
+    def __repr__(self): ...
+```
+
+```python
+@dataclass(frozen=True)
+class SignedGaussCode:
+    components: list[list[SignedGaussCodeCrossing]]
+
+    def writhe(self): ...
+    def reverse(self): ...
+    def mirror(self): ...
+    def to_std_unknot(self) -> SignedGaussCode: ...
+    def std_unknot_switching_sequence(self) -> list[int]: ...
+    def apply_switching_sequence(self, seq: list[int]) -> SignedGaussCode: ...
+    def splice_h(self, id: int): ...
+    def splice_v(self, id: int): ...
+    def switch_crossing(self, id: int): ...
+
+    def __repr__(self): ...
+```
+
+==== Writhe
+
+One of the first important things we need is to compute the *writhe* $w(K)$ of a link, this can easily be done with signed gauss codes as its a list of tuples where the second entry is the crossing sign. Let $L$ be an oriented link with components $C_1, ..., C_k$ each with crossings $c_(i, j)$ with $i = 1, ..., k$ and $j = 1, ..., |C_i|$.
+
+Let's notice that here each crossing appears twice, once as over-crossing and once as an under-crossing this is the reason for the $1 slash 2$ in the following formula. By $epsilon(c)$ we refer to the sign (or handedness) of the crossing at $c$.
+
+#[
+  #set align(center)
+  #grid(
+    columns: 3,
+    gutter: 1.5em,
+    align: horizon,
+    [
+      $
+        w(L) = 1 / 2 sum_(c "crossing") epsilon(c)
+      $
+    ],
+    [$ arrow.squiggly $],
+    [
+      ```python
+      def writhe(self):
+          return sum(
+              c.handedness # => +1 or -1
+              for component in self.components
+              for c in component
+          ) // 2
+      ```
+    ],
+  )
+]
+
+==== Standard Unknot
+
+The next building block for computing the Kauffman polynomial is detecting and computing the *standard unknot or unlink*. Formally this is done by taking the _planar shadow_ and a directed starting point on it. Then we can walk along the shadow and make each crossing an over-crossing when passing on it on the first time.
+
+On the other hand our algorithm directly works with switching sequences $lambda$ that bring a knot $K$ to its standard unknot $hat(K)$. We wrote methods to directly compute and apply these switching sequences.
+
+```python
+def std_unknot_switching_sequence(self) -> list[int]:
+    visited_crossings: set[int] = set()
+    switched_crossings: list[int] = []
+
+    for component in self.components:
+        for crossing in component:
+            if crossing.id not in visited_crossings:
+                if crossing.is_under():
+                    switched_crossings.append(crossing.id)
+                visited_crossings.add(crossing.id)
+
+    return switched_crossings
+```
+
+The `std_unknot_switching_sequence` method just walks along each component in its orientation marking what switches have to be made to bring that link to its standard unknot.
+
+
+```python
+def apply_switching_sequence(self, seq: list[int]) -> SignedGaussCode:
+    return SignedGaussCode(
+      [
+        [
+            crossing.opposite() if crossing.id in switching_sequence else crossing
+            for crossing in component
+        ]
+        for component in self.components
+      ]
+    )
+```
+
+Applying a switching sequence is just a matter of walking along the crossings and flipping the crossings that are in the sequence. This is also how the `switch_crossing(id: int)` method works.
+
+==== Crossing Splices
+
+The splicing code is more involved due to the number of cases to analyze, let's first see formally what we need to do.
+
+We have all the following cases, first we can assume the _entering over strand_ is in the top left corner of a diagram (this can be done by applying locally a small isotopy). So we have $2$ cases for the crossing sign
+
+#{
+  set align(center)
+
+  grid(
+    columns: 4,
+    grid(
+      //
+      columns: 5,
+      gutter: 1.5em,
+      align: horizon,
+
+      skein-generic(kind: "over", direction: (+1, +1)),
+      skein-generic(kind: "over", direction: (+1, -1)),
+      [$arrow.squiggly$],
+      skein.h,
+      [$arrow.squiggly$],
+
+      skein-generic(kind: "over", direction: (+1, +1)),
+      skein-generic(kind: "over", direction: (+1, -1)),
+      [$arrow.squiggly$],
+      skein.v,
+      [$arrow.squiggly$],
+    ),
+  )
+}
+
+
+
+So the final code is just a conversion of all this cases to list slicing and re-joining with the appropriate crossings removed.
+
+```python
+def splice_h(self, id: int):
+    raise NotImplementedError("Splicing not implemented yet")
+
+def splice_v(self, id: int):
+    raise NotImplementedError("Splicing not implemented yet")
+```
+
 #pagebreak()
 
 = Appendix

theme.typ

@@ -114,6 +114,7 @@
     } else {
       set text(size: 10pt, fill: luma(20%))
       v(normal-size * 2, weak: true)
+      [$thin diamond.medium space$]
       strong(it.body)
       v(normal-size * 1.5, weak: true)
     }