# [Public Notes] Low-Diameter Decomposition (LDD) By Danupon Nanongkai **Goal: outline the proof of a theorem about directed Low-Diameter Decomposition (LDD) from [BNW22] (Theorem 1 below).** Remarks: * These notes have not been subjected to the usual scrutiny reserved for formal publications. * The algorithm presented here is slightly different from [BNW22] but the main ideas are the same. Due to this, there might be some errors in these notes. * While I tried to make these notes self-contained, the main purpose is to supplement my lecture. * I usually start with precise mathematical statements, followed by the discussions. If you are lost in the math, look at the discussions for some help. * Let me know if more explanations are needed somewhere. ## Standard definitions Come back to this section if you see some definitions you don't recognize. * Throughout, we consider directed graphs $G = (V,E,w)$ with integer edge weight function $w$. Let $m:=|E|$ and $n:=|V|$. We also use $V(G)$ to denote the sets of vertices in $G$. * Let $G[S]$ denote the subgraph of $G$ induced by $S\subseteq V$. * For any $V'\subseteq V$ and $E'\subseteq E$, define $G\setminus V'$ and $G\setminus E'$ as the graphs resulting from removing vertices in $V'$ and edges in $E'$, respectively. * $dist_G(u,v)$ = the total weight of the shortest path path from $u$ to $v$ in $G$. * DAG = Directed Acycle Graph * W.h.p. = with high probability * $\tilde O$ hides $polylog(n)$. ## 1. Main theorem **Definition (weak/strong diameter):** Given a weighted directed graph $G = (V,E,w)$ and $S\subseteq V$, the **weak diameter** (w.r.t. $G$) of $S$ or $G[S]$ is defined as $\max_{u,v\in S} dist_G(u,v)$. Their **strong diameter** is defined as $\max_{u,v\in S} dist_{G[S]}(u,v)$. --- **Theorem 1:** There is a randomized algorithm $LDD(G,D)$ with the following guarantees: * INPUT: an $m$-edge $n$-vertex directed graph $G = (V,E,w)$ with non-negative integer edge weight function $w$ and a positive integer $D$. * OUTPUT: An edge set $E_{Rem}\subseteq E$ such that 1. each SCCs of $G\setminus E_{rem}$ has weak diameter $O(D)$, i.e. if $u, v$ are in the same SCC, then $dist_G(u, v)=O(D)$ and $dist_G(v, u)=O(D),$ and 3. for every $e\in E$, $Pr[e\in E_{rem}]=O\left(\frac{w(e) \cdot (\log n)^2}{D} + n^{-8}\right)$. * RUNTIME: The algorithm has running time $\tilde O(m+n)$ in expectation. --- **Remarks:** It is **not** guaranteed that the events $e\in E_{rem}$ and $e'\in E_{rem}$ are independent, for any $e,e'\in E$. [BNW22] uses $E_{sep}$ instead of $E_{rem}$. ("Rem" here stands for "Remove".) **Example (can skip):** If $G$ is an undirected path $(v_1, v_2, ..., v_n)$, then we can achieve the theorem by randomly selecting $i\in [1, D]$ and adding edges $(v_i, v_{i+1})$, $(v_{i+D}, v_{i+D+1})$, $(v_{i+2D}, v_{i+2D+1})$, ... to $E_{rem}$. ## 2. Undirected LDD via ball growing We start with a classic algorithm for computing a LDD for **undirected** graphs. We will prove output properties similar to Theorem 1. We will not discuss the runtime yet. ### 2.1 Algorithm **Definition (Ball):** For any vertex $s$ in an *undirected* graph $G$ and postive integer $R$, let $Ball_G(s,R)=\{v\in V \mid dist_G(s,v)\leq R\}.$ Let $\partial Ball_G(s,R)=\{(x,y)\in E \mid x\in Ball_G(s,R), y\notin Ball_G(s,R)\}$ --- **Algorithm** UndiLDD(G, D) <span style="color:gray">// Explanation is below</span> 1. $E_{rem}\leftarrow\emptyset$. 2. While $G$ is not empty: 2.1 Pick arbitrary vertex $v$ in $G$ 2.2 Sample $R_v\sim Geom(p)$ for $p=\min\{1, 20\ln(n)/D\}.$ 2.3 If $R_v> D/2$, let $R_v=D/2.$ 2.4 Add edges in $\partial Ball_G(v,R_v)$ to $E_{rem}$. <span style="color:gray">// Make $Ball_G(v,R_v)$ a $SCC$ in $G\setminus E_{rem}.$</span> 2.5 $G\leftarrow G\setminus Ball_G(v,R_v).$ **Return** $E_{rem}$. --- **Explanation (can skip):** The algorithm carves out a "ball" of some radius around some vertex $v$. It then repeats this until the graph is empty. The balls form a vertex partition and $E_{rem}$ is simply the set of edges between them. The key is to pick the radius according to a geometric distribution with parameter $p\approx 1/D$, as in step 2.2 in the pseudocode. This means that for any $x\geq 1$, $Pr[R_v = x] = (1 - p)^{x-1} p$. This is equivalent to the number of time we need to flip a coin until it turns head, which happens with probability $p\approx 1/D$. **Ball growing process:** The algorithm can be viewed as starting with a ball of radius 1 at some vertex $v$. We flip a coin that turns head with probability $p\approx 1/D$. Every time we get a tail, we increase the radius of the ball by one. We stop when we get a head or the radius is $D$. We cut out the ball and repeat the ball growing process from a new vertex. **Example:** Let $G$ be an undirected path $(v_1, v_2, ..., v_n)$. The algorithm might do the following. Start with $v_1$ and pick $R_v\in [1, D/2]$ as in step 2.2. Add edge $(v_{R_v+1}, v_{R_v+2})$ to $E_{rem}$. Remove nodes $v_1, ..., v_{R_v+1}$ and repeat the process with $v_{R_v+2}$ treated as $v_1$. ### 2.2 Output Property 2 **Lemma 2.1:** For every $e\in E$, $Pr[e\in E_{rem}] = O\left(\frac{w(e) \cdot (\log n)}{D}+n^{-9}\right).$ **Proof sketch:** **(I)** Consider the algorithm *without* step 2.3, i.e. we allow $R_v>D$. We claim that $Pr[e\in E_{rem}]\leq w(e)\cdot p$$= O\left(\frac{w(e) \cdot (\log n)}{D}\right) *Slightly informal proof:* * Consider any edge (u,v). Consider the first time one of u and v (say u) is contained in a ball during the ball-growing process above. Until this time (u,v) has zero chance of being in E_{rem}. * After this, either (i) both of u and v are in the ball or (ii) edge (u,v) is added to E_{rem}. * If w(u,v)=1, whether (i) or (ii) occurs depends on the next coin flip (what happened before this does not matter). In particular, if we get head (happening w.p. p), the process stops and e\in E_{rem}; otherwise, both u and v are in the ball so e will never be in E_{rem}. So, Pr[e\in E_{rem}]\leq p. * This argument requires the **memoryless property**; e.g., if R_v is uniform in [1,D] and the ball's radius is currently D, then Pr[e\in E_{rem}]=1. * For w(e)> 1, the same idea gives Pr[e\in E_{rem}]\leq w(e)\cdot p. *More formal proof:* See [here](https://hackmd.io/@U0nm1XUhREKPYLt1eUmE6g/BJ1k5dr35) **(II)** Now consider the effect of step 2.3: Some edge (u,v) might be added to E_{rem} when we set R_v=D in Step 2.3. But the probability that step 2.3 is ever executed is very small: * For any vertex v, Pr[R_v\geq D/2]$$\leq (1-p)^{D/2}\leq e^{-pD/2}$$=e^{-\frac{20 \ln(n) D}{2D}}=n^{-10}$ * Note: $\forall x>1$, $(1-1/x)^{x-1}>1/e>(1-1/x)^x$. * By Union Bound, the probability that we set $R_v=D$ in Step 2.3, causing any edge to be added to $E_{rem}$ is $\leq n^{-9}$. **QED** *Remark:* The proof above crucially requires the memoryless property of the geometric random variable, i.e. $Pr[R_v\geq i+j | R_v\geq i]=Pr[R_v\geq j]$. Exercise: Argue that Theorem 1 does not hold if $R_v$ is selected uniformly from $[1,D]$ instead. ### 2.3 Output Property 1 Output property 1 in Theorem 1 is easy to prove: Observe that the SCCs (i.e. the connected components) of $G\setminus E_{rem}$ are the balls that we carved out. Since we carved out balls of radius $R_v\leq D$, each SCC has strong(!) diameter $O(D)$. *Remark:* Observe that we get slightly stronger output properties than stated in Theorem 1: we get a strong diameter of $O(D)$ and $Pr[e\in E_{rem}]\approx \frac{w(e) \cdot (\log n)}{D}$ instead of $\frac{w(e) \cdot (\log n)^2}{D}$. ## 3. Directed LDD via light nodes & recursion We now extend the above algorithm to directed graphs. We will not discuss the runtime yet. ### 3.1 Highlights (can skip) Details are in later sections. * For digraph, our balls are $Ball^{in}_G(s, R)$ and $Ball^{out}_G(s, R)$. We perform ball carving process like above. * Detail: For each carved ball, we include edges only in *one* direction to $E_{rem}$ (either pointing out of or into the ball). * We carve out only balls centered at $v$ with $Ball^{in}_G(v, D)$ or $Ball^{out}_G(v, D)$ of size $\leq n/2$. Call such $v$ a (in/out-)**light node**. Nodes that are not in-light and out-light are called **heavy nodes**. * **Key lemma:** The set of heavy nodes have weak diameter $O(D)$. * **Recursion:** We repeat the ball-carving framework on the carved-out balls. Eventually, each component becomes a singleton or has low diameter. Since we recurse on a ball of size $\leq n/2$, the probability that an edge is in $E_{rem}$ is $O(\log(n))$ times more than the undirected case. ### 3.2 Algorithm **Definition (in/out-Balls):** For any vertex $s$ in a *directed* graph $G$ and postive integer $R$, let $Ball^{out}_G(s,R)=\{v\in V \mid dist_G(s,v)\leq R\}$ and $Ball^{in}_G(s,R)=\{v\in V \mid dist_G(v,s)\leq R\}.$ Let $\partial Ball_G^{out}(s,R)=\{(x,y)\in E \mid x\in Ball_G(s,R), y\notin Ball_G(s,R)\}$ and $\partial Ball_G^{in}(s,R)=\{(y,x)\in E \mid x\in Ball_G(s,R), y\notin Ball_G(s,R)\}$ **Example:** The red nodes (big circles) in the picture below are in $Ball^{in}_G(v,2).$ The blue (dashed) edges are in $\partial Ball^{in}_G(v,2).$ ![](https://i.imgur.com/B9XDuPN.png =150x) --- **Algorithm** LDD($G$, $D$) <span style="color:gray">// See explanation below</span> 1. $E_{rem}\leftarrow\emptyset$ and $n\leftarrow |V(G)|$. <span style="color:gray">// $n$ does not change in the steps below.</span> 2. For every $v\in V$, if $|Ball^{in}_G(v, D)|\leq n/2$, mark $v$ as **in-light**; else if $|Ball^{out}_G(v, D)|\leq n/2$, mark $v$ as **out-light**. 3. While $G$ contains node $v$ marked **in-light**: 3.1 Sample $R_v\sim Geom(p)$ for $p=\min\{1, 20\log(n)/D\}$. If $R_v> D/2$, let $R_v=D/2$. 3.2 $E_{rem}'\leftarrow LDD\left(G[Ball_G^{in}(v,R_v)], D\right)$. <span style="color:gray">// Recurse</span> 3.3 $E_{rem}\gets E_{rem} \cup \partial Ball_G^{in}(v,R_v) \cup E'_{rem}$ 3.4 $G\leftarrow G\setminus Ball_G^{in}(v,R_v)$. 4. While $G$ contains node $v$ marked **out-light**: <span style="color:gray">// Repeat step 3 with "out" instead of "in" </span> 4.1 Sample $R_v\sim Geom(p)$ for $p=\min\{1, 20\log(n)/D\}$. If $R_v> D$, let $R_v=D$. 4.2 $E_{rem}''\leftarrow LDD\left(G[Ball_G^{out}(v,R_v)], D\right)$. <span style="color:gray">// Recurse</span> 4.3 $E_{rem}\gets E_{rem} \cup \partial Ball_G^{out}(v,R_v) \cup E''_{rem}$ 4.4 $G\leftarrow G\setminus Ball_G^{out}(v,R_v)$. **Return** $E_{rem}$ --- **Explanation:** Since $G$ is directed, we distinguish the balls to "in-balls" $Ball^{out}_G(s,R)$ and "out-balls" $Ball^{in}_G(s,R)$, where $Ball^{out}_G(s,R)$ include nodes whose distance **from** $s$ is at most $R$ and $Ball^{in}_G(s,R)$ include nodes whose distance **to** $s$ is at most $R$. For simplicity, let's assume that $G$ is strongly-connected. If we ignore the recursions (steps 3.2 and 4.2), the algorithm (steps 3-4) simply carves out from $G$ balls of random radius as before. However, it does so only for in/out-**light** nodes defined in step 2. (We will show below that the remaining graph already has $O(D)$ weak diameter.) Another difference is that it only adds edges in one direction to $E_{rem}$: If it carves out an in-ball $Ball_G^{in}(v,R_v)$, then it adds edges pointing **into** the ball to $E_{rem}$ (step 3.3, where we set $E_{rem}\gets E_{rem} \cup \partial Ball_G^{in}(v,R_v)$). Similarly, if it carves out an out-ball $Ball_G^{out}(v,R_v)$, then it adds edges pointing **out from** the ball (step 4.3). Another difference to the undirected case is the **recursion**: Since we cannot guarantee that the balls we carved out have low diameter, we recursively decompose them (steps 3.2 and 4.2). Since these balls contain $\leq n/2$ nodes, intuitively this increases the probability that an edge is in $E_{rem}$ by a factor of $O(\log n)$. Note that the recursion stops when, e.g., $n=1$ since there is no light node. **Example:** Let $G$ be a path on $V=\{v_1, ..., v_n\}$ with edges $(v_i, v_{i+1})$ and $(v_{i+1}, v_i)$ for every $i$. Let $D=\sqrt{n}$. Observe that every node is in- and out-light. The algorithm may start with $Ball^{in}_G(v_1, R_{v_1})$. Since $\partial Ball^{in}_G(v_1, R_{v_1})=\{(v_{i+1}, v_{i})\}$ where $i=R_{v_1}+1$, the algorithm adds $(v_{i+1}, v_{i})$ to $E_{rem}$. Ignoring the recursion for now, the algorihtm proceeds by removing $v_1, v_2, \ldots, v_{R_{v_1}+1}$ from $G$ and repeat the process at, say, $v_{i+1}$. Some more edges will be added to $E_{rem}$ during this. The algorithm also recurses on the subgraph induced by $V'=\{v_1, v_2, \ldots, v_{R_{v_1}+1}\}.$ No nodes in $V'$ will be marked in/out-light since for any $v \in V'$, $Ball^{in}_{G[V']}(v, D)=Ball^{out}_{G[V']}(v, D)=V'$. So the recursion returns $E_{rem}=\emptyset.$ Eventually, we get a result similar to the picture below where blue (dashed) edges are in $E_{rem}$. ![](https://i.imgur.com/annSgYT.png) ### 3.3 Output Property 1 (weak diameter) In each recursive call of the algorithm, we call nodes marked in- and out-light in step 2 as **light nodes** and call other nodes **heavy**. **Key lemma:** The set of heavy nodes have weak diameter $\leq 2D$, i.e. any two heavy nodes $u,v\in V(G)$, $dist_G(u,v)\leq 2D$ **Proof:** * Consider any two heavy nodes $u,v\in V(G)$. * There exists $x\in Ball^{out}_G(u, D)\cap Ball^{in}_G(v, D)$ * since $Ball^{out}_G(u, D)>n/2$ and $Ball^{in}_G(v, D)>n/2.$ * Thus, $dist_G(u,v)\leq dist_G(u,x)+dist_G(x,v)\leq 2D$. **QED** The key lemma easily implies property 1. Detail: * Let $B_i$, for $i=1, 2, \ldots,$ be the $i^{th}$ carved-out ball. Let $B_\infty$ be the set of vertices that remain in $G$ after we carve out all balls in Step 3-4. * Any $x\in B_i$ and $y \in B_j$ are **not** in the same SCC in $G\setminus E_{rem}$ for any $i<j$. * This is because we either put to $E_{rem}$ all edges pointing into or out of $B_i$ * e.g. if $B_i=Ball^{out}_G(v, R_v)$ for some vertex $v$, then we put edges in $\partial Ball^{out}_G(v, R_v)$ to $E_{rem}$ and thus there is no path from $x$ to $y$ in $G\setminus E_{rem}$. * Thus, each SCC of $G\setminus E_{rem}$ is contained in some $B_i$. SCCs in $B_\infty$ has $O(D)$ weak diameter by the key lemma. We can argue by induction that other SCCs has $O(D)$ weak diameter. ### 3.4 Output Property 2 Output property 2 simply follows from By the analysis from the unweighted case (Lemma 2.1) and the fact that each edge participates in $O(\log n)$ recursive calls. Detail: * By the analysis from the unweighted case (Lemma 2.1), in each recursive call LDD($G$, $D$), for every edge $E\in E(G)$, $Pr[e\in E_{rem}]= O\left(\frac{w(e) \cdot (\log n)}{D}+n^{-9}\right)$. * Each edge participates in $O(\log n)$ recursive calls; i.e. any edge $e$ in the input graph is contained in $O(\log n)$ among all recursive calls of LDD($G$, $D$). * Reason: When we make recursive calls $LDD\left(G[Ball_G^{in}(v,R_v)], D\right)$ and $LDD\left(G[Ball_G^{out}(v,R_v)], D\right)$ in steps 3.2 and 4.2, the graphs $G[Ball_G^{in}(v,R_v)]$ and $G[Ball_G^{out}(v,R_v)]$ contain $\leq n/2$ vertices. (The recursive calls stop when $n=1$ at the latest.) * By union bound, for every edge $e$ in the input graph, $Pr[e\in E_{rem}]= O\left(\frac{w(e) \cdot (\log n)^2}{D}+n^{-9}\log n\right)= O\left(\frac{w(e) \cdot (\log n)^2}{D}+n^{-8}\right)$. **Exercise:** Modify the algorithm to guarantee $O(D)$ *strong* diameter. (Hint: Keep carving out balls from nodes that are light in the current graph.) ## 4. Runtime **Highlights (can skip):** The most interesting part is detecting light nodes (step 2). Main ideas: * We estimate $|Ball^{in}_G(v, D)|$ and $|Ball^{out}_G(v, D)|$ by sampling a set $S$ of $\Theta(\log n)$ nodes and compute $|Ball^{in}_G(v, D)\cap S|$ and $|Ball^{out}_G(v, D)\cap S|.$ * We can compute $|Ball^{in}_G(v, D)\cap S|$ and $|Ball^{out}_G(v, D)\cap S|$ for *all* nodes $v$ in $\tilde O(m)$ time in total by computing $Ball^{in}_G(s, D)$ and $Ball^{out}_G(s, D)$ for every $s\in S$. * The algorithm and proof in the previous section can be easily modified to handle the fact that we only know the estimates of $|Ball^{in}_G(v, D)|$ and $|Ball^{out}_G(v, D)|$. ### 4.1 Detecting light nodes (step 2) #### 4.1.1 Implementing step 2: 2.1 $k\gets c\ln n$ for a big enough constant $c$. 2.2 $S\gets \{s_1, s_2, \ldots s_k\}$ where $s_i$ is a random node in $V$ for every $i$. * Possible: $s_i=s_j$ for some $i\neq j$. 2.3 For each $s\in S$, compute $Ball^{in}_G(s, D)$ and $Ball^{out}_G(s, D)$ <span style="color:gray">// $O(mk)$ time </span> 2.4 For each $v\in V$: <span style="color:gray">// $O(nk)$ time </span> * $|Ball^{in}_G(v, D)\cap S|\gets\{s\in S\mid v\in Ball^{out}_G(s, D)\}$ * $|Ball^{out}_G(v, D)\cap S|\gets \{s\in S\mid v\in Ball^{in}_G(s, D)\}$ 2.5 For every $v\in V$, if $|Ball^{in}_G(v, D)\cap S|\leq (0.6)k$, mark $v$ as **in-light**; else if $|Ball^{out}_G(v, D)\cap S|\leq (0.6) k$, mark $v$ as **out-light**. --- #### 4.1.2 Chernoff's bound: The following lemma easily follows from Chernoff's bound. **Lemma 4.1:** W.h.p., the following holds for every $v\in V$ and $*\in \{in, out\}$: $\frac{|Ball^{*}_G(v, D)\cap S|}{k}- 0.1 \leq \frac{|Ball^{*}_G(v, D)|}{n} \leq \frac{|Ball^{*}_G(v, D)\cap S|}{k} + 0.1$ **Proof (tedious):** We use the following form of Chernoff's bound. * Let $π_1, . . . , π_k$: independent random variables s.t. $πβ€πΏ_πβ€π$ for all $π$. * Let $π_{ππ£π} =(1/k) \sum_{i=1}^k π_π$ * For any $π>0$, (a) $Prβ‘[π_{ππ£π}>πΈ[π_{ππ£π} ]+π]β€π^{β2kπ^2}$ (b) $Prβ‘[π_{ππ£π}<πΈ[π_{ππ£π} ]βπ]β€π^{β2kπ^2}$ Consider any $v\in V$: * For any $i\in [1,k]$, define $X_i=1$ if $s_i\in Ball^{in}_G(v, D)$ and $X_i=0$ otherwise. * $E[X_i]=|Ball^{in}_G(v, D)|/n$ $\forall i$. * So, $X_{avg}=\frac{|Ball^{in}_G(v, D)\cap S|}{k}$ and $E[X_{avg}]=\frac{|Ball^{in}_G(v, D)|}{n}$. * By (a), $Pr\left[\frac{|Ball^{in}_G(v, D)\cap S|}{k}>\frac{|Ball^{in}_G(v, D)|}{n}+0.1\right]\leq 1/n^{10}$. In other words, w.h.p., $\frac{|Ball^{in}_G(v, D)|}{n}\geq \frac{|Ball^{in}_G(v, D)\cap S|}{k}-0.1.$ Other inequalities are proved similarly. The lemma follows by union bound. **QED** ---- #### 4.1.3 Output Properties (sketched): Lemma 4.1 implies that the following holds w.h.p. **(I)** For any $*\in \{in,out\}$, if $|Ball^{*}_G(v, D)\cap S|>0.6k$, then $|Ball^{*}_G(v, D)|>0.5n$. Thus, w.h.p., if $v\in V$ is *not* marked in- and out-light in step 2.5, then $|Ball^{in}_G(v, D)|>0.5n$ and $|Ball^{out}_G(v, D)|>0.5n$. Consequently, we can use the key lemma in the previous section to argue that the weak diameter of every SCC in $G\setminus E_{rem}$ is $O(D)$ w.h.p. **(II)** For any $*\in \{in,out\}$, if $|Ball^{*}_G(v, D)\cap S|\leq 0.6k$, then $|Ball^{*}_G(v, D)|\leq 0.7n$. This implies that, w.h.p., we only recurses on balls of size $\leq 0.7n$. This implies that, w.h.p., the recursion depth is $O(\log n)$ like before. Thus the proof in section 3 can still be used to show that $Pr[e\in E_{rem}]=O\left(\frac{w(e) \cdot (\log n)^2}{D} + n^{-8}\right)$. **Eliminating "whp":** Above we argue that the output properties in Theorem 1 hold w.h.p. We can avoid this "w.h.p.": * We check if the weak diameter of every SCC is $O(D)$ * by computing whether the distances from some node $v$ in the SCC to all other nodes; they must be $O(D)$. * We can check if we always recurse on balls of size $\leq 0.7n$. * If one of the above does not hold, we put every edge in $E_{rem}$. This happens with probability (say) $\leq 1/n^{10}$, so it does not affect our guarantee on $Pr[e\in E_{rem}]$. ### 4.2 Runtime of steps 3-4 * Generating random variables (steps 3.1 & 4.1): $O(\log n)$ expected runtime if we use [BF13]. * By Theorem 5 in [BF13] with $p=\log(n)/D$ and word size $w=\Omega(\log\log D)$, the expected time is $O(\log(\min\{D/\log n, n\})/\log w)=O(\log n)$ * There might be other simpler methods since we do not need an exact algorithm. Most programming languages also provide highly optimized methods to do this (e.g. numpy.random.geometric). * Since we remove $Ball_G^{in}(v,R_v)$ and $\partial Ball_G^{in}(v,R_v)$ from $G$ after we compute them in steps 3.2-3.3, the total time require by steps 3.2-3.4 is $O(m)$. The same holds for steps 4.3-4.4. So, ignoring the recursion, steps 3-4 require $O(m)$ time in total. ### 4.3 Summary The runtime of step 2 is $O(mk)=O(m\log n)$ and steps 3-4 requires $O(m)$ runtime. Since each edge participates in $O(\log n)$ recursions, the total runtime is $O(m(\log n)^2)$. Finally, for clarity, below is the pseudocode of the fast LDD algorithm we discussed (we combine the previous steps 3-4 for compactness). **Algorithm** FastLDD($G$, $D$) <span style="color:gray">// See explanation below</span> 1. $G_0\gets G$. $E_{rem}\leftarrow\emptyset$ and $n\leftarrow |V(G)|$. <span style="color:gray">// $n$ does not change in the steps below.</span> 2. For each $v\in V$: 2.1 $k\gets c\ln n$ for a big enough constant $c$. 2.2 $S\gets \{s_1, s_2, \ldots s_k\}$ where $s_i$ is a random node in $V$ for every $i$. (Possible: $s_i=s_j$ for some $i\neq j$.) 2.3 For each $s\in S$, compute $Ball^{in}_G(s, D)$ and $Ball^{out}_G(s, D)$ <span style="color:gray">// $O(mk)$ time </span> 2.4 For each $v\in V$: <span style="color:gray">// $O(nk)$ time </span> * $|Ball^{in}_G(v, D)\cap S|\gets\{s\in S\mid v\in Ball^{out}_G(s, D)\}$ * $|Ball^{out}_G(v, D)\cap S|\gets \{s\in S\mid v\in Ball^{in}_G(s, D)\}$ 2.5 For every $v\in V$, if $|Ball^{in}_G(v, D)\cap S|\leq (0.6)k$, mark $v$ as **in-light**; else if $|Ball^{out}_G(v, D)\cap S|\leq (0.6) k$, mark $v$ as **out-light**. 3. While $G$ contains node $v$ marked **$*$-light** for any $*\in \{in,out\}$: 3.1 Sample $R_v\sim Geom(p)$ for $p=\min\{1, 20\log(n)/D\}$. If $R_v> D/2$, let $R_v=D/2$. 3.2 Compute $Ball_G^{*}(v,R_v)$. If $|Ball_G^{*}(v,R_v)|>0.7n$, return $E_{rem}=E_{rem}\cup E(G)$ and terminate. <span style="color:gray">// Claim: $Pr[terminate]\leq 1/n^{10}$</span> 3.3 $E_{rem}'\leftarrow LDD\left(G[Ball_G^{*}(v,R_v)], D\right)$. <span style="color:gray">// Recurse</span> 3.4 $E_{rem}\gets E_{rem} \cup \partial Ball_G^{*}(v,R_v) \cup E'_{rem}$ 3.4 $G\leftarrow G\setminus Ball_G^{*}(v,R_v)$. 4. Let $v$ be any node in $G$. If $dist_{G_0}(v,u)>2D$ or $dist_{G_0}(u,v)>2D$ for any node $u$, return $E_{rem}=E_{rem}\cup E(G)$ and terminate. <span style="color:gray">// Claim: $Pr[terminate]\leq 1/n^{10}$</span> * (If $dist_{G_0}(v,u)\leq 2D$ and $dist_{G_0}(u,v)\leq 2D$, we can conclude that the weak diameter of $G$ w.r.t. $G_0$ is $\leq 4D$.) **Return** $E_{rem}$ ## Historical notes As far as I know: * Undirected LDD was first used by Awerbuch in the context of distributed network synchronizers. * Baruch Awerbuch: Complexity of network synchronization, J. ACM 32(1985), 804-823. * The algorithm for undirected graphs presented here was due to Linial and Saks. * Nathan Linial and Michael Saks. Low diameter graph decompositions. Combinatorica, 13(4):441β454, 1993 * This algorithm was used by Bartal to construct a probabilistic tree embedding algorithm. Because of this, the LDD algorithm for undirected graphs has been widely taught. * Yair Bartal: Probabilistic Approximations of Metric Spaces and Its Algorithmic Applications. FOCS 1996: 184-193 * Undirected LDDs were used to construct many important graph-theoretic structures like probabilistic tree embeddings (mentioned above), oblivious routings, hopsets, neighborhood covers, etc. These structures are crucially used in the state-of-the-art algorithms in many models of computation, e.g., for [group Steiner tree approximations](https://www.sciencedirect.com/science/article/abs/pii/S0196677400910964) (via tree embeddings), [various balanced cut problems](https://dl.acm.org/doi/abs/10.1145/1374376.1374415) (via oblivious routing), shortest path (via [hopsets](https://dl.acm.org/doi/abs/10.1145/331605.331610) and [neighborhood covers](https://dl.acm.org/doi/abs/10.1145/3519935.3520074)). Applications spread over many models of computation (e.g. parallel and distributed settings). See the two papers below for some surveys. * Gary L Miller, Richard Peng, and Shen Xu. Parallel graph decompositions using random shifts. Proc. of the Symposium on Parallelism in Algorithms and Architectures (SPAA), pages 196β203, 2013. * Sebastian Forster, Martin GrΓΆsbacher, Tijn de Vos: An Improved Random Shift Algorithm for Spanners and Low Diameter Decompositions. OPODIS 2021: 16:1-16:17 * A recent breakthough include deterministic parallel and distributed algorithms to compute a structure similar to undirected LDDs by [Rozhon, Ghaffari](https://arxiv.org/abs/1907.10937). * The directed version was first developed to solve the dynamic shortest paths problem by the paper below. The algorithm doesn't take near-linear time but already contains basic ideas that lead to the near-linear time bound above. * Aaron Bernstein, Maximilian Probst Gutenberg, and Christian Wulff-Nilsen. Near-optimal decremental SSSP in dense weighted digraphs. FOCS 2020. ## Acknowledgement I would like to thank Parinya Chalermsook, Mohsen Ghaffari, Thatchaphol Saranurak, Goran Zuzic, and participants at the EPFL summer school 2022 for comments. The formal proof of Lemma 2.1(1) benefits from the discussions with Parinya. Section 4 has been simplified based on the discussions with Mohsen. ## References [BF13] Karl Bringmann, Tobias Friedrich: Exact and Efficient Generation of Geometric Random Variates and Random Graphs. ICALP (1) 2013: 267-278 [link](https://people.mpi-inf.mpg.de/~kbringma/paper/2013ICALP-1.pdf) [BNW22] Negative-Weight Single-Source Shortest Paths in Near-linear Time. [arXiv](https://arxiv.org/abs/2203.03456)